[clang] [OpenMP][CIR] Implement basic 'parallel' lowering + some clause infra (PR #172308)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Note: I've got 1 more patch after this one (and the barrier one) that will show how a clause can be implemented, but it is dependent on this one.  `proc_bind` seemed easy enough to implement, so I'll submit that after this gets merged.

However, that'll be my last CIR/OMP patch.

https://github.com/llvm/llvm-project/pull/172308