[benchmarks] Move libcxx's fork of google/benchmark and llvm/utils'

under third-party

This change:
- moves the libcxx copy of `google/benchmark` to
`third-party/benchmkark`
- points the 2 uses of the library (libcxx and llvm/utils) to this copy

We picked the licxx copy because it is the most up to date.

Differential Revision: https://reviews.llvm.org/D112012