[clang] [llvm] [clang] Add regalloc PBQP for all targets in clang (PR #166645)

[Ivan Shumakov via llvm-commits]

Zararest wrote:

@lhames could you, please, review these changes or give a hint who I should tag
(My tag is based on a mention in the llvm discussion from the first comment of this PR)

https://github.com/llvm/llvm-project/pull/166645