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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Okay, just for clarity, CIR codegen is the lowering from CIR to LLVM dialect MLIR, right?
> 

this change is something I requested specifically for the purpose of additional duplication.

The specific goal of the PR is to start reducing the code duplication for the IR agnostic parts of the ABI.

The existing code already has a lot of duplication, and this is just the start of of deduplication.

ie this PR is reducing the duplication rather than increasing it.

that said I would like to see if we can start sharing more of the IR generator - maybe a hypothetical IRBuilder interface?

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