[llvm] [llvm][ABI] Add X86_64 ABI classifier (PR #194433)

[via llvm-commits]

vortex73 wrote:

@adams381, the upstreaming for this library is going on in breakout PRs. As I converyed in our email discussion, the x86_64 classifier will be merged soon - this time is to come up with a proper way to test it prior to merging, given the complexity. In the meanwhile upstreaming is still underway for the rest of the clang integration: https://github.com/llvm/llvm-project/pull/194460

https://github.com/llvm/llvm-project/pull/194433