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

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

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

I was matching the name of the psuedo-instruction. I'll change it to `.seh_startepilogue`.

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

I don't really know how we'd share it - the ARM implementation is trivial, but it's use of `TargetStreamer` instead of deriving from `MCStreamer` makes it difficult to share code.

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