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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203950 91177308-0d34-0410-b5e6-96231b3b80d8