[benchmark] Pull latest; disable -Werror

Pulled the latest (b000672), which adds an option to disable -Werror,
which caused issues previously (see for instance
https://reviews.llvm.org/D112012)

Applied https://github.com/google/benchmark/pull/1305 to unblock, as
this issue is causing downstream buildbot failures.

The only change is in llvm/CMakeLists.txt, where we set
BENCHMARK_ENABLE_WERROR to have the same value as LLVM_ENABLE_WERROR.

This patch also introduces a convenience script to pull benchmark from
upstream.

Differential Revision: https://reviews.llvm.org/D115684