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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I've rebased this PR to bring in recent changes, fixed a few problems that were caused by recent changes, and generally refactored the command-line handling to align with a similar change that was made in the incubator.

This doesn't lower the ops and types that have been added since this PR was originally created. I'll handle that in a follow-up PR.

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