[DeclChecker] Avoid setting ErrorType on the enum cases with duplicate names

We want to preserve as much of the AST as possible in the valid state,
so for cases with duplicate names it makes sense to mark declaration
itself and its parent enum as invalid and leave the original types in place.

Author: xedin

lib/Sema/TypeCheckDecl.cpp

@@ -4522,10 +4522,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         for (auto *EED : ED->getAllElements()) {
           auto Res = Elements.insert({ EED->getName(), EED });
           if (!Res.second) {
-            EED->setInterfaceType(ErrorType::get(TC.Context));
             EED->setInvalid();
-            if (auto *RawValueExpr = EED->getRawValueExpr())
-              RawValueExpr->setType(ErrorType::get(TC.Context));
 
             auto PreviousEED = Res.first->second;
             TC.diagnose(EED->getLoc(), diag::duplicate_enum_element);
@@ -4535,10 +4532,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
           }
         }
 
-        if (hasErrors) {
-          ED->setInterfaceType(ErrorType::get(TC.Context));
+        // If one of the cases is invalid, let's mark
+        // whole enum as invalid as well.
+        if (hasErrors)
           ED->setInvalid();
-        }
       }
     }