[clang][Sema] Use enumerator instead of hard-coded constant

Sema::DiagnoseSwiftName uses the constant 12 instead of the
corresponding enumerator ExpectedFunctionWithProtoType. This is
fragile and will fail if a new value gets added in the middle of the
enum.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D88164

Author: miyuki

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5868,7 +5868,7 @@ bool Sema::DiagnoseSwiftName(Decl *D, StringRef Name, SourceLocation Loc,
 
       if (!F->hasWrittenPrototype()) {
         Diag(Loc, diag::warn_attribute_wrong_decl_type) << AL
-            << /* non-K&R-style functions */12;
+            << ExpectedFunctionWithProtoType;
         return false;
       }
     }