<rdar://problem/13267210> Ensure that Sema::CompareReferenceRelationship returns consistent results with invalid types.

When Sema::RequireCompleteType() is given a class template
specialization type that then fails to instantiate, it returns
'true'. On subsequent invocations, it can return false. Make sure that
this difference doesn't change the result of
Sema::CompareReferenceRelationship, which is expected to remain stable
while we're checking an initialization sequence.