[clang-tools-extra] [clangd] Implement simple folding for preprocessor branches (PR #140959)

[Nathan Ridge via cfe-commits]

================
@@ -220,6 +220,24 @@ getFoldingRanges(const std::string &Code, bool LineFoldingOnly) {
   auto EndPosition = [&](const Token &T) {
     return offsetToPosition(Code, EndOffset(T));
   };
+
+  // Preprocessor directives
+  auto PPRanges = pairDirectiveRanges(DirectiveStructure, OrigStream);
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HighCommander4 wrote:

Could you add a testcase that has a nested conditional in both branches of an outer conditional, and checks that both nested conditionals get a folding range? (It would be easy to get this wrong by passing `ParseableStream` rather than `OrigStream` here, which would have the effect of only processing the branch of the outer conditional which we heuristically decided to treat as active.)

https://github.com/llvm/llvm-project/pull/140959