[llvm] [win][x64] Unwind v2 4/4: Use chaining to split unwind info if needed, and allow dealloc to be elided if setframe + frame pointer is used (PR #159206)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Please add a test that emits the `.seh_splitchained` assembly directive.

https://github.com/llvm/llvm-project/pull/159206