[clang] [CIR] Initial implementation of CIR-to-LLVM IR lowering pass (PR #125260)

[Erich Keane via cfe-commits]

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

LGTM, but Bruno has an open, and I would love more details to one of the comments (re-flat CIR) if it is a sensible request.

https://github.com/llvm/llvm-project/pull/125260