[CSSimplify] Check that raw type is not a hole before checking RawRepresentable conformance

Placeholder types just like type variables are set up to return
a conformance ref which, in this case, would mean that we'd produce
an invalid diagnostic.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5093,7 +5093,7 @@ bool ConstraintSystem::repairFailures(
     if (auto objType = valueType->getOptionalObjectType())
       valueType = objType;
 
-    if (rawReprType->isTypeVariableOrMember())
+    if (rawReprType->isTypeVariableOrMember() || rawReprType->isPlaceholder())
       return false;
 
     auto rawValue = isRawRepresentable(*this, rawReprType);