[clang-tools-extra] [clangd] Add clarifying comment about when Dex::IdxContents is populated (PR #118906)

[Nathan Ridge via cfe-commits]

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@@ -121,6 +121,8 @@ class Dex : public SymbolIndex {
   llvm::DenseMap<std::pair<SymbolID, uint8_t>, std::vector<SymbolID>> Relations;
   std::shared_ptr<void> KeepAlive; // poor man's move-only std::any
   // Set of files which were used during this index build.
+  // Files and IdxContents are only populated for dynamic and background
+  // indexes, not static indexes.
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HighCommander4 wrote:

That's a fair point.

My motivation with this patch is to avoid the sort of confusion I had when looking at https://github.com/llvm/llvm-project/pull/118324, where I was wondering if maybe the fact that some codepaths do not propagate a value for `IdxContents` is an oversight.

I revised the comment to avoid talking about the specifics of higher layers here, let me know if that looks better.

https://github.com/llvm/llvm-project/pull/118906