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

[Martin Storsjö via cfe-commits]

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mstorsjo wrote:

We do have some level of ARM SEH support since a couple years: We can generate unwind info, which is enough for using SEH for C++ unwinding on ARM mingw (which uses itanium C++ ABI for the C++ ABI stuff).

But the MSVC style C constructs, `__try`, `__except` are not supported yet, and neither is MSVC C++ unwinding. I tried to have a brief look at implementing them at the same time, but it was less trivial than I had hoped for, so I deferred that as I didn't have any need for that.

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