[ADT] Increment epoch from DenseMap::swap.

Swapping DenseMap A with DenseMap B invalidates iterators pointing into
both A and B.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233890 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/DenseMap.h

@@ -584,6 +584,8 @@ public:
   }
 
   void swap(DenseMap& RHS) {
+    this->incrementEpoch();
+    RHS.incrementEpoch();
     std::swap(Buckets, RHS.Buckets);
     std::swap(NumEntries, RHS.NumEntries);
     std::swap(NumTombstones, RHS.NumTombstones);