Refactor: Simplify boolean conditional return statements in llvm/lib/Support

Summary: Use clang-tidy to simplify boolean conditional return statements

Reviewers: rafael, bkramer, ddunbar, Bigcheese, chandlerc, chapuni, nicholas, alexfh

Subscribers: alexfh, craig.topper, llvm-commits

Patch by Richard Thomson!

Differential Revision: http://reviews.llvm.org/D9978

llvm-svn: 256500