[llvm] [ADT][TableGen] Add UniquingSet, a FoldingSet with typed keys (PR #219630)

[Kazu Hirata via llvm-commits]

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@@ -159,6 +160,19 @@ template <typename T, typename Ctx> struct DefaultContextualFoldingSetTrait {
 template <typename T, typename Ctx>
 struct ContextualFoldingSetTrait : DefaultContextualFoldingSetTrait<T, Ctx> {};
 
+class FoldingSetInsertPos {
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kazutakahirata wrote:

May I suggest something like this?
```suggestion
/// An opaque token returned by UniquingSet::find on a lookup miss, recording
/// the computed hash so that insert() can place the new node without re-hashing.
class FoldingSetInsertPos {
```

https://github.com/llvm/llvm-project/pull/219630