[clang-tools-extra] [clang-doc] fix flaky test in clang-doc (PR #101387)

[Paul Kirth via cfe-commits]

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@@ -104,6 +104,9 @@ struct Reference {
 
   bool mergeable(const Reference &Other);
   void merge(Reference &&I);
+  bool operator<(const Reference &Other) const {
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ilovepi wrote:

How stable are usrs? Perhaps we should be sorting on another property?

https://github.com/llvm/llvm-project/pull/101387