[Clang][CodeGen] support alias attribute w/ gnu_inline

Summary:
r369705 did not consider the addition of gnu_inline on function
declarations of alias attributed functions. This resulted in a reported
regression in the clang-9-rc4 release from the Zig developers building
glibc, which was observable as a failed assertion:

llvm-project/clang/lib/AST/Decl.cpp:3336: bool
clang::FunctionDecl::isInlineDefinitionExternallyVisible() const:
Assertion `(doesThisDeclarationHaveABody() || willHaveBody()) && "Must
be a function definition"' failed.

Alias function declarations do not have bodies, so allow us to proceed
if we have the alias function attribute but no body/definition, and add
a test case.  The emitted symbols and their linkage matches GCC for the
added test case.

Link: https://bugs.llvm.org/show_bug.cgi?id=43268

Reviewers: aaron.ballman, rsmith, erichkeane, andrewrk

Reviewed By: andrewrk

Subscribers: cfe-commits, andrewrk, hans, srhines

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67455

llvm-svn: 371766

clang/lib/AST/Decl.cpp

@@ -3348,7 +3348,8 @@ SourceRange FunctionDecl::getExceptionSpecSourceRange() const {
 /// an externally visible symbol, but "extern inline" will not create an
 /// externally visible symbol.
 bool FunctionDecl::isInlineDefinitionExternallyVisible() const {
-  assert((doesThisDeclarationHaveABody() || willHaveBody()) &&
+  assert((doesThisDeclarationHaveABody() || willHaveBody() ||
+          hasAttr<AliasAttr>()) &&
          "Must be a function definition");
   assert(isInlined() && "Function must be inline");
   ASTContext &Context = getASTContext();

clang/test/CodeGen/alias.c

@@ -99,3 +99,8 @@ static int test10_foo __attribute__((alias("test10")));
 // CHECKGLOBALS-NOT: @test11_foo = dso_local
 void test11(void) {}
 static void test11_foo(void) __attribute__((alias("test11")));
+
+// Test that gnu_inline+alias work.
+// CHECKGLOBALS: @test12_alias = alias void (), void ()* @test12
+void test12(void) {}
+inline void test12_alias(void) __attribute__((gnu_inline, alias("test12")));