[clang-tools-extra] [clangd] Support go-to-definition on type hints. The core part (PR #86629)

[Nathan Ridge via cfe-commits]

================
@@ -55,6 +55,19 @@ struct ExpectedHint {
   }
 };
 
+struct ExpectedHintLabelPiece {
+  std::string Label;
+  std::optional<std::string> PointsTo = std::nullopt;
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HighCommander4 wrote:

It's not immediately clear what this stores, let's add:

```
// Stores the name of a range annotation
```

https://github.com/llvm/llvm-project/pull/86629