[CS] Replace UnresolvedType with ErrorType in simplifyType/resolveType

This means we now either produce a bare ErrorType, or an ErrorType
with a generic parameter original type for a generic parameter hole.
We ought to further consolidate this logic by sinking the generic
parameter original type replacement into `simplifyType` itself, but
I'm leaving that for a future patch since it affects completion
results and I want to try keep this close to NFC.

lib/IDE/SelectedOverloadInfo.cpp

@@ -56,7 +56,7 @@ swift::ide::getSelectedOverloadInfo(const Solution &S,
         OverloadChoiceKind::KeyPathApplication) {
       auto Params = Result.ValueTy->getAs<AnyFunctionType>()->getParams();
       if (Params.size() == 1 &&
-          Params[0].getPlainType()->is<UnresolvedType>()) {
+          Params[0].getPlainType()->is<ErrorType>()) {
         auto *KPDecl = CS.getASTContext().getKeyPathDecl();
         Type KPTy =
             KPDecl->mapTypeIntoContext(KPDecl->getDeclaredInterfaceType());