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

[miyuki]

Summary

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.

[aaron.ballman]

Good catch, LGTM!

clang/lib/Sema/SemaDeclAttr.cpp

if (isa<ObjCMethodDecl>(D) || isa<FunctionDecl>(D)) {
     } else {
       const auto *F = cast<FunctionDecl>(D);
 
       ParamCount = F->getNumParams();
       Params = F->parameters();
 
       if (!F->hasWrittenPrototype()) {
         Diag(Loc, diag::warn_attribute_wrong_decl_type) << AL
-            << /* non-K&R-style functions */12;
+            << ExpectedFunctionWithProtoType;
         return false;
       }
     }
 
     unsigned SwiftParamCount;
     bool IsSingleParamInit;
     if (!validateSwiftFunctionName(*this, AL, Loc, Name,
                                    SwiftParamCount, IsSingleParamInit))