[clang] [CIR] Add framework for CIR to LLVM IR lowering (PR #124650)

[Bruno Cardoso Lopes via cfe-commits]

https://github.com/bcardosolopes approved this pull request.

Seems like there are some CI failures (format related?) but LGTM once those are fixed

https://github.com/llvm/llvm-project/pull/124650