[clang] [Clang] Fix `PPChainedCallbacks::EmbedFileNotFound()` (PR #170293)

[Nathan Corbyn via cfe-commits]

cofibrant wrote:

I've added a unit test, but had to make a small tweak to the logic in `HandleDirective()` to avoid unwrapping an empty optional 

https://github.com/llvm/llvm-project/pull/170293