[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:

My concern here is that we probably want clang-doc to output to be ordered consistently. While sorting by USRs may make it deterministic, it’s also likely that they won’t be ordered similarly run to run. For instance, I’d expect public class fields to be sorted by name. 

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