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

[Ruihua Dong via cfe-commits]

144026 wrote:

@zyn0217  Thanks for the information, seems like a completed removal of pseudo parser has been decided in this [discourse thread](https://discourse.llvm.org/t/removing-pseudo-parser/71131), and shifting back to AST parser for foldingRange support would only lose a marginal multi-line comment folding feature.

Dependency of this patch hasn't been touched yet in #80081 , so I tested a rebased build of this patch on #80081 on my Linux box, which still could pass and work.

I understand that evolving this end-of-life parser only adds maintenance burden and should be avoided, but I hope there will be some ways (may be in the original AST parser?) to support PP conditional folding, especially when reading complex headers.

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