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

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> ... I'd like to see at least a test added to this PR.

See #100556. The test would fail with assertions enabled (thus, marked XFAIL: asserts), and when compiled against hardened libc++ (e.g. libc++ built with `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST`).

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