Revert "[Concepts] Placeholder constraints and abbreviated templates"
This reverts commit e57a9abc4b.
Parser/cxx2a-placeholder-type-constraint.cpp has MSan failures.
Present at 7b81c3f8793d30a4285095a9b67dcfca2117916c:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap-msan/builds/17133/steps/check-clang%20msan/logs/stdio
not present at eaa594f4ec54eba52b03fd9f1c789b214c66a753:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap-msan/builds/17132/steps/check-clang%20msan/logs/stdio
Stack trace:
```
==57032==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0xccfe016 in clang::AutoTypeLoc::getLocalSourceRange() const /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/include/clang/AST/TypeLoc.h:2036:19
#1 0xcc56758 in CheckDeducedPlaceholderConstraints(clang::Sema&, clang::AutoType const&, clang::AutoTypeLoc, clang::QualType) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:4505:56
#2 0xcc550ce in clang::Sema::DeduceAutoType(clang::TypeLoc, clang::Expr*&, clang::QualType&, llvm::Optional<unsigned int>, bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:4707:11
#3 0xcc52407 in clang::Sema::DeduceAutoType(clang::TypeSourceInfo*, clang::Expr*&, clang::QualType&, llvm::Optional<unsigned int>, bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:4457:10
#4 0xba38332 in clang::Sema::deduceVarTypeFromInitializer(clang::VarDecl*, clang::DeclarationName, clang::QualType, clang::TypeSourceInfo*, clang::SourceRange, bool, clang::Expr*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaDecl.cpp:11351:7
#5 0xba3a8a9 in clang::Sema::DeduceVariableDeclarationType(clang::VarDecl*, bool, clang::Expr*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaDecl.cpp:11385:26
#6 0xba3c520 in clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Sema/SemaDecl.cpp:11725:9
#7 0xb39c498 in clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseDecl.cpp:2399:17
#8 0xb394d80 in clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::SourceLocation*, clang::Parser::ForRangeInit*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseDecl.cpp:2128:21
#9 0xb383bbf in clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::Parser::ParsedAttributesWithRange&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseDecl.cpp:1848:10
#10 0xb383129 in clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::Parser::ParsedAttributesWithRange&, clang::SourceLocation*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/llvm/include/llvm/ADT/PointerUnion.h
#11 0xb53a388 in clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::Parser::ParsedAttributesWithRange&) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseStmt.cpp:221:13
#12 0xb539309 in clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseStmt.cpp:106:20
#13 0xb55610e in clang::Parser::ParseCompoundStatementBody(bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseStmt.cpp:1079:11
#14 0xb559529 in clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseStmt.cpp:2204:21
#15 0xb33c13e in clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/Parser.cpp:1339:10
#16 0xb394703 in clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::SourceLocation*, clang::Parser::ForRangeInit*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseDecl.cpp:2068:11
#17 0xb338e52 in clang::Parser::ParseDeclOrFunctionDefInternal(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec&, clang::AccessSpecifier) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/Parser.cpp:1099:10
#18 0xb337674 in clang::Parser::ParseDeclarationOrFunctionDefinition(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*, clang::AccessSpecifier) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/Parser.cpp:1115:12
#19 0xb334a96 in clang::Parser::ParseExternalDeclaration(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/Parser.cpp:935:12
#20 0xb32f12a in clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/Parser.cpp:686:12
#21 0xb31e193 in clang::ParseAST(clang::Sema&, bool, bool) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Parse/ParseAST.cpp:158:20
#22 0x80263f0 in clang::FrontendAction::Execute() /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Frontend/FrontendAction.cpp:936:8
#23 0x7f2a257 in clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:965:33
#24 0x8288bef in clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:290:25
#25 0xad44c2 in cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/tools/driver/cc1_main.cpp:239:15
#26 0xacd76a in ExecuteCC1Tool(llvm::ArrayRef<char const*>) /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/tools/driver/driver.cpp:325:12
#27 0xacc9fd in main /b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm-project/clang/tools/driver/driver.cpp:398:12
#28 0x7f7d82cdb2e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
#29 0xa4dde9 in _start (/b/sanitizer-x86_64-linux-bootstrap-msan/build/llvm_build_msan/bin/clang-11+0xa4dde9)
```
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