[clang] [CIR][CodeGen][NFCI] Move shared AST-only helpers into CodeGenUtils (PR #223418)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I spot checked and this seems reasonable?  But it moves a LOT of things from classic codegen, so I am not going to approve it since I think final approval should come from @efriedma-quic .

https://github.com/llvm/llvm-project/pull/223418