Fix a crash (assertion failure) in EvaluateAsRValue.

Summary:
Gracefully fail to evaluate a constant expression if its type is
unknown, rather than failing an assertion trying to access the type.

Reviewers: klimek

Reviewed By: klimek

CC: chandlerc, cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3075

llvm-svn: 203950