Summary:
Use the cache action to cache the clang build instead of the
facebook-clang-plugins releases.
- the facebook-clang-plugins repo is no longer by infer so its clang
will soon be out of date
- much simpler to use the cache action
- build clang before setting opam: this is unfortunate but needed to
ensure there is enough space on GitHub's machines to build clang...
- install ninja on Linux + Mac machines for faster and less OOM-y builds
Reviewed By: martintrojer
Differential Revision: D24160876
fbshipit-source-id: 705e06151