[clang] OpenMP: Reimplement getOffloadArch (PR #189561)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Does this work with `--offload-arch=gfx950:+xnack` or something like that?

I have no idea what's going on here. I'm guessing this is a PTX hack, given the `hasFeature("ptx")` check. But that also raises the question of why ptx would have to have a ptx feature 

https://github.com/llvm/llvm-project/pull/189561