[clang] [llvm] [mlir] [CIR] Expand callee-side va_arg on x86_64 (PR #222420)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

The new helper functions and cursor handling is a nice improvement. I still have some concerns about alignment.

https://github.com/llvm/llvm-project/pull/222420