[clang] [llvm] [AArch64][COFF] Branch-protection=standard/pac-ret means "b-key" on Windows (PR #203989)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

@DanielKristofKiss As this is approved, and CI is passing, can we merge this now?

We'll need to request a backport to 23.x, and RC1 is meant to be tagged imorgon, so it'd be nice to have it merged there as soon as possible.

https://github.com/llvm/llvm-project/pull/203989