[clang] Avoid accessing unset optional, workaround for #100095 (PR #100408)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM, since it now has a test that executes this code path and changes from UB to defined (even if undesired behavior).

As mentioned earlier, I'm working on a proper fix for those cases

https://github.com/llvm/llvm-project/pull/100408