[Sema] Don't crash when diagnosing hack in libstdc++

While working around a bug in certain standard library implementations,
we would try to diagnose the issue so that library implementors would
fix their code.  However, we assumed an entity being initialized was
a non-static data member subobject when other circumstances are
possible.

This fixes PR24526.

llvm-svn: 245675
2 files changed