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

[Nico Weber via cfe-commits]

nico wrote:

e272c37934a06cd80b9b072afc09afae5fd8c218 might have fixed this. (Didn't test locally, will check what the bot says.)

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