Sema: Don't introduce ErrorType when an associated type is witnessed by an Objective-C generic parameter

In general it's fine to emit a diagnostic but leave the conformance
valid here, we're not going to go on to emit code, and an ErrorType
here causes other diagnostics to be not be emitted in a test due
to minor validation order perturbations.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -1910,8 +1910,7 @@ void ConformanceChecker::recordTypeWitness(AssociatedTypeDecl *assocType,
 
   assert(!type->hasArchetype() && "Got a contextual type here?");
 
-  if (checkObjCTypeErasedGenerics(assocType, type, typeDecl))
-    type = ErrorType::get(type);
+  checkObjCTypeErasedGenerics(assocType, type, typeDecl);
 
   if (typeDecl) {
     // Check access.