[clang] [llvm] [LLVMABI] Implement the System V X86-64 ABI (PR #194718)

[Nikita Popov via cfe-commits]

https://github.com/nikic approved this pull request.

LGTM. I haven't reviewed the code in detail this round, but other people have, and the remaining failures in consistency checks are edge cases. I think we can go ahead and land this to unblock CIR, and do remaining minor fixups afterwards. (That will also be easier to review than one big patch.)

https://github.com/llvm/llvm-project/pull/194718