[clang-tools-extra] [clangd] Implement simple folding of preprocessor branches (rebased) (PR #121449)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

This is my first time looking at this code (SemanticSelection.cpp / DirectiveTree.cpp) so I'm not sure if I'm the best reviewer, but on a high level what this patch is doing seems reasonable to me.

Some unit tests for the change would be great. We have some existing unit tests for folding ranges in [SemanticSelectionTests.cpp](https://searchfox.org/llvm/rev/1bd5f34d76b788de48e95d2f9c1179cf2685fbe5/clang-tools-extra/clangd/unittests/SemanticSelectionTests.cpp) which you can extend.

https://github.com/llvm/llvm-project/pull/121449