[clang] [mlir] [CIR] Add sret, byval, and struct flattening to CIRABIRewriteContext (PR #192124)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

This is pretty huge. Would it be possible to break it up and implement one type at a time?

https://github.com/llvm/llvm-project/pull/192124