[clang] [WinEH] Check object unwinding in seh block only when -eha is used (PR #180108)

[Hans Wennborg via cfe-commits]

zmodem wrote:

I'm not up-to-date with the full context here. Can you add to the description that this is a followup to #172287?

Maybe silly question, but why is this specific to `/EHa` and not having C++ exceptions enabled in general (like `/EHs`)?

Also the PR should probably have a test.

https://github.com/llvm/llvm-project/pull/180108