[llvm] [llvm] Win x64 Unwind V2 1/n: Mark beginning and end of epilogs (PR #110024)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The existing ARM/AArch64 backends use `.seh_startepilogue`; is there some reason you chose to use `.seh_beginepilogue` here instead?

It looks like this doesn't touch the existing parsing codepaths for `.seh_startepilogue`/``.seh_endepilogue`; can we share those across targets?

https://github.com/llvm/llvm-project/pull/110024