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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman any gripes with moving forward and landing this after a month?

Nope, I think it's fine to move ahead with this. If @MaskRay has concerns, they can be addressed post commit.

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