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

[Kazu Hirata 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());
----------------
kazutakahirata wrote:

Both this test and the one in `StandaloneInfoAliasingKeyAcrossGrowth` only test `getOrInsert` with a node that is already present in the set. Could we also test the case where the node is absent, verifying that `getOrInsert` inserts the new node, returns it, and increments `size()`?

For example, at the end of `TEST(UniquingSetTest, Basic)`:

```cpp
  KeyedPair C(3, 4);
  EXPECT_EQ(&C, Set.getOrInsert(&C));
  EXPECT_EQ(2u, Set.size());
  EXPECT_EQ(&C, Set.lookup({3, 4}, Token));
```

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