[clang] [Clang][CodeGen][CIR][NFC] Refactor CGCXXABI classes for code sharing (PR #165078)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

Following discussions at the LLVM Developer Meeting, I believe we reached a consensus that the type of refactoring strategy pursued in this PR might be premature and could prove to be unnecessary after we implement (in the possibly distant future) some of the ideas we discussed. That being the case, I think it's better to tolerate a significant degree of code duplication as we drive toward a working CIR implementation with the explicit understanding that we intend to do something about it in the long term.

I'm preparing a document describing the general direction we discussed. I'll put it up for review soon to be available in the clang/docs directory.

https://github.com/llvm/llvm-project/pull/165078