[clang] [OpenMP][CIR] Add basic infrastructure for CIR lowering (PR #171902)

[Erich Keane via cfe-commits]

erichkeane wrote:

Note: I do NOT intend to do the OpenMP lowering for CIR, however as I had some cycles, I figured I'd start putting together a little bit of infrastructure for whoever WANTS to do it to use as a guide.  This is a 1st of 3 patches that will do SOME level of lowering for OMP (after which point, I would expect/need someone else to take over for this).

@razvanlupusoru : if you could help tag some folks who are doing the OMP Lowering in Flang so they are at least aware of this, it would be GREATLY appreciated.

https://github.com/llvm/llvm-project/pull/171902