Introduce a Bazel build configuration

This patch introduces configuration for a Bazel BUILD in a side
directory in the monorepo.

This is following the approval of
https://github.com/llvm/llvm-www/blob/main/proposals/LP0002-BazelBuildConfiguration.md

As detailed in the README, the Bazel BUILD is not supported
by the community in general, and is maintained only by interested
parties. It follows the requirements of the LLVM peripheral tier:
https://llvm.org/docs/SupportPolicy.html#peripheral-tier.

This is largely copied from https://github.com/google/llvm-bazel,
with a few filepath tweaks and the addition of the README.

Reviewed By: echristo, keith, dblaikie, kuhar

Differential Revision: https://reviews.llvm.org/D90352