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

[Kazu Hirata via llvm-commits]

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@@ -389,6 +403,11 @@ class FoldingSetBase : public DebugEpochBase {
   /// it is not already in the folding set.  InsertPos must be obtained from
   /// FindNodeOrInsertPos for an ID that \p N profiles identically to.
   LLVM_ABI void InsertNode(Node *N, void *InsertPos);
+
+  /// Encode \p Hash, which must not be NotAHash, as the token InsertNode takes.
+  static void *encodeInsertPos(uint32_t Hash) {
+    return reinterpret_cast<void *>(static_cast<uintptr_t>(Hash));
+  }
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kazutakahirata wrote:

Could we have both `encodeInsertPos` and `decodeInsertPos` here and then remove the duplicate in `FoldingSet.cpp`?

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