[clang] Clang/Preprocessor: Support short circuit in directive (PR #123912)

[Michael Spencer via cfe-commits]

Bigcheese wrote:

It's an alternative to https://github.com/llvm/llvm-project/pull/120673

This resolves my concerns with the original patch, and I'm pretty sure it doesn't cause any other issues, but I do plan to take a closer look in a bit.

https://github.com/llvm/llvm-project/pull/123912