[clang] [llvm] [ARM] Add support for Windows SEH (PR #184953)

[Trung Nguyen via cfe-commits]

trungnt2910 wrote:

@kristibektashi Thanks for the deep insights! 

That said, my current use case is just kernel-mode SEH, so I guess non-Thumb is not really involved, similar to how C++ `try`/`catch` is not in scope for this PR.

I believe the reviewers, myself, and the agents behind me all agree that we should keep this PR as small as meaningfully possible. 

Right now I am addressing the feedback - these are real problems, and I am trying to fix them as well as ensuring that the related tests pass. 

https://github.com/llvm/llvm-project/pull/184953