[clang] [CIR] Upstream the basic structure of LoweringPrepare pass (PR #148545)

[Bruno Cardoso Lopes via cfe-commits]

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

We need to update some of the entries because they are old now, so a few comments on that, otherwise LGTM

https://github.com/llvm/llvm-project/pull/148545