[clang] [CIR] Add CIRGenUtils.h for shared CIR codegen utilities (PR #172735)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

This is basically what I had in mind. I was thinking of it as specific to the builtin handling, but I suppose there is no reason it needs to be.

This function in particular has been a bit slippery. I like it as a helper function. It feels like it could just be moved to CIRGenFunction, but if we did that we'd have to pass `cgf` around to a lot of other helper functions that otherwise only need the builder.

@HendrikHuebner You did a lot of the work to cleanup the builtin handling code, including introducing this function. What do you think about this?

https://github.com/llvm/llvm-project/pull/172735