[clang-tools-extra] [clangd] Implement simple folding of preprocessor branches (PR #80592)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Note that clang-pseudo is deprecated and will soon get [removed](https://github.com/llvm/llvm-project/pull/80081) from the repository. I have no idea about the replacement, and I'm not sure if it makes sense to evolve the feature with it. 

I invited some folks (who may be in charge of clang-pseudo) for visibility.

https://github.com/llvm/llvm-project/pull/80592