Sema: Make applyGenericArguments() idempotent

If we call TypeRepr::setInvalid() we need to return an ErrorType, because
we'll return an ErrorType if this same TypeRepr is resolved again.

This just makes the RequirementMachine's behavior match the
GenericSignatureBuilder on some highly-invalid compiler_crashers.

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -679,7 +679,7 @@ static Type applyGenericArguments(Type type, TypeResolution resolution,
 
       generic->setInvalid();
     }
-    return type;
+    return ErrorType::get(ctx);
   }
 
   auto *unboundType = type->castTo<UnboundGenericType>();