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

[Trung Nguyen via llvm-commits]

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trungnt2910 wrote:

How should I demonstrate that in a test case? Since C++ EH is not implemented for `armv7-pc-windows-msvc` (even the most simple example would [crash](https://godbolt.org/z/Mze3YoYb6) the compiler), what should I use for the `CHECK` lines?

For the other tests, there are already some checks to ensure that `finally` accesses go through `r1` in `seh-finally.ll`. Does this sound good:
- Add more `CHECK` lines in `ehcleanup` to assert that `r6` got moved to `r1`.
- Add a clone of this test (maybe `seh-except.ll`) that asserts that `r6` is used in the filter and `__except` funclets in the typical case and in scenarios where VLAs and realigning are involved.


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