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

[Erich Keane via llvm-commits]

erichkeane wrote:

> > This has been repeatedly asked over the last ~month and a half. As Adam received no response, we had to conclude that Narayan had no real preference.
> 
> I'm not on the email chain but I'm told (by Narayan) that he responded fairly promptly to the outreach (even if not immediately). Maybe something was lost in the "tubes" but regardless, Narayan is here _now_ communicating his preferences and plans.

Also not on the chain, though Adam tells me the responses, while debatably prompt, didn't answer the questions asked (particularly: how can we help, etc).

That said, I'm glad Narayan is communicating!  Adam, Andy, and I are looking forward to helping with this project, as I said, it is in a bit of a critical path for the Clang-IR effort, which is important to the FE for obvious reasons.

I think it is smarter to take the rest of this offline.  @adams381 : can you copy Maksim + me + Andy on the email chain so we can make sure Adam/us can help get this progressed faster without stepping on  toes?

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