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

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> Can you please add a reference to #100095 in the summary so folks just reading the git log have more context w/o going to the commit itself.
> 
> I would like to see a more flushed out long-term plan for fixing this properly but I think it makes sense.

I've added more details to the commit message and another reference to #100095 (one was already in the short summary).

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