[clang] initialize type qualifiers for FunctionNoProtoType

When initializing FunctionNoProtoType types, zero out the type
qualifiers. This will ensure the ODR hash remains stable as it
hashes the values for these qualifiers for all function types.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D133586

clang/include/clang/AST/Type.h

@@ -3892,7 +3892,10 @@ protected:
   }
 
   Qualifiers getFastTypeQuals() const {
-    return Qualifiers::fromFastMask(FunctionTypeBits.FastTypeQuals);
+    if (isFunctionProtoType())
+      return Qualifiers::fromFastMask(FunctionTypeBits.FastTypeQuals);
+
+    return Qualifiers();
   }
 
 public:

clang/unittests/AST/DeclTest.cpp

@@ -354,3 +354,25 @@ TEST(Decl, FriendFunctionWithinClassInHeaderUnit) {
 
   EXPECT_TRUE(getFooValue->isInlined());
 }
+
+TEST(Decl, NoProtoFunctionDeclAttributes) {
+  llvm::Annotations Code(R"(
+    void f();
+    )");
+
+  auto AST = tooling::buildASTFromCodeWithArgs(
+      Code.code(),
+      /*Args=*/{"-target", "i386-apple-darwin", "-x", "objective-c",
+                "-std=c89"});
+  ASTContext &Ctx = AST->getASTContext();
+
+  auto *f = selectFirst<FunctionDecl>(
+      "f", match(functionDecl(hasName("f")).bind("f"), Ctx));
+
+  const auto *FPT = f->getType()->getAs<FunctionNoProtoType>();
+
+  // Functions without prototypes always have 0 initialized qualifiers
+  EXPECT_FALSE(FPT->isConst());
+  EXPECT_FALSE(FPT->isVolatile());
+  EXPECT_FALSE(FPT->isRestrict());
+}