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

[Nico Weber via cfe-commits]

nico wrote:

Oh, I think it's just the old ` clang: warning: '/Users/thakis/src/llvm-project/clang/test/Driver/windows-seh-async-verify.cpp' treated as the '/U' option [-Wslash-u-filename]`. I'll try pushing a fix.

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