[Clang][Sema] Check the return value of DiagnoseClassNameShadow in ActOnEnumConstant

Static analysis flagged that we were not checking the return value of
DiagnoseClassNameShadow when we did so everywhere else. Modifying this case
to match how other places uses it makes sense and does not change behavior.
Likely if this check fails later actions will fail as well but it is more
correct to exit early.

Author: shafik

clang/lib/Sema/SemaDecl.cpp

@@ -20134,9 +20134,10 @@ Decl *Sema::ActOnEnumConstant(Scope *S, Decl *theEnumDecl, Decl *lastEnumConst,
   // different from T:
   // - every enumerator of every member of class T that is an unscoped
   // enumerated type
-  if (getLangOpts().CPlusPlus && !TheEnumDecl->isScoped())
-    DiagnoseClassNameShadow(TheEnumDecl->getDeclContext(),
-                            DeclarationNameInfo(Id, IdLoc));
+  if (getLangOpts().CPlusPlus && !TheEnumDecl->isScoped() &&
+      DiagnoseClassNameShadow(TheEnumDecl->getDeclContext(),
+                              DeclarationNameInfo(Id, IdLoc)))
+    return nullptr;
 
   EnumConstantDecl *New =
     CheckEnumConstant(TheEnumDecl, LastEnumConst, IdLoc, Id, Val);