[clang] [SEH] Ignore async exception flag when the environment is not MSVC (PR #88101)

[Nico Weber via cfe-commits]

nico wrote:

Looks like breaks check-clang on macOS: http://45.33.8.238/macm1/83520/step_6.txt

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/88101