[llvm] [ADT] Make DenseMapBase::swap the public entry point (PR #167650)

[Jakub Kuderski via llvm-commits]

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@@ -360,6 +360,12 @@ class DenseMapBase : public DebugEpochBase {
     return getBuckets();
   }
 
+  void swap(DerivedT &RHS) {
+    this->incrementEpoch();
+    RHS.incrementEpoch();
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kuhar wrote:

Is the epoch number something we could test for?

https://github.com/llvm/llvm-project/pull/167650