[llvm] [ADT] Document the UniquingSet contracts. NFC (PR #221860)

[Fangrui Song via llvm-commits]

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@@ -635,6 +635,10 @@ TEST(UniquingSetTest, Basic) {
   KeyedPair B(2, 1);
   Set.insert(&B, Token);
 
+  KeyedPair ADup(1, 2);
+  EXPECT_EQ(&A, Set.getOrInsert(&ADup));
+  EXPECT_EQ(2u, Set.size());
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MaskRay wrote:

Improved unittest!

https://github.com/llvm/llvm-project/pull/221860