[clang] [CIR] Initial implementation of lowering CIR to MLIR (PR #127835)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

We agreed at the clangir upstreaming meeting today that this PR should be put on hold. We don't intend to keep lowering to MLIR standard dialects up-to-date as we upstream the rest of the implementation, and it is currently missing a lot of functionality in the incubator, so it seems better to leave this for a future effort.

https://github.com/llvm/llvm-project/pull/127835