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

[Hans Wennborg via cfe-commits]

zmodem wrote:

I understand that you're working on improving the support for async SEH. That's great, and it sounds like Chromium has code which will benefit.

However, you didn't answer my main question: if `err_seh_object_unwinding` is for diagnosing mixing SEH and C++ exception handling, why did this PR gate it behind `getLangOpts().EHAsynch` and not `getLangOpts().CXXExceptions`?

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