Fix a build warning of unhandled enum in switch

Summary: LLVM adds a new value FMRB_DoesNotReadMemory in the enumeration.

Reviewers: andrew.w.kaylor, chrisj, zinob, grosser, jdoerfert

Subscribers: Meinersbur, pollydev

Differential Revision: http://reviews.llvm.org/D22109

llvm-svn: 275085