[llvm] [ADT] Make DenseMapBase::moveFrom safer (NFC) (PR #168180)

[Jakub Kuderski via llvm-commits]

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@@ -1107,6 +1118,13 @@ class SmallDenseMap
     this->BaseT::initEmpty();
   }
 
+  // Put the zombie instance in a known good state after a move.
+  void kill() {
+    deallocateBuckets();
+    Small = false;
+    new (getLargeRep()) LargeRep{nullptr, 0};
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kuhar wrote:

could we make it small and avoid allocating a large state?

https://github.com/llvm/llvm-project/pull/168180