[clang] [CIR] Build out AST consumer patterns to reach the entry point into CIRGen (PR #91007)

[Aaron Ballman via cfe-commits]

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

My concerns have been addressed, but there's still a small open question on the driver flag behavior though, so please give @MaskRay a chance to respond before landing.

Otherwise, LGTM!

https://github.com/llvm/llvm-project/pull/91007