Sema: Don't crash when trying to instantiate a local class with an invalid base specifier

It was previously thought that Sema::InstantiateClass could not fail
from within this point in instantiate.

However, it can happen if the class is invalid some way (i.e. invalid
base specifier).

This fixes PR18907.

Differential Revision: http://llvm-reviews.chandlerc.com/D2850