Add experimental support for multi-threading the pass manager. This adds...

Add experimental support for multi-threading the pass manager. This adds support for running function pipelines on functions across multiple threads, and is guarded by an off-by-default flag 'experimental-mt-pm'. There are still quite a few things that need to be done before multi-threading is ready for general use(e.g. pass-timing), but this allows for those things to be tested in a multi-threaded environment.

PiperOrigin-RevId: 240489002