[benchmark] Silence warning by applying upstream patch

ompiling benchmark library (introduced in D50894) with the latest
bootstrapped Clang produces a lot of warnings, this issue was addressed
in the upstream patch I pushed earlier.

Upstream patch:
https://github.com/google/benchmark/commit/f85304e4e3a0e4e1bf15b91720df4a19e90b589f

`README.LLVM` notes were updated to reflect the latest changes.

Reviewed by: lebedev.ri

Differential Revision: https://reviews.llvm.org/D51342

llvm-svn: 340811