[ADT] Remove dead code.

Nothing is using DenseMapBase::swap.  Besides it does not compile in its
current form.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233889 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/DenseMap.h

@@ -341,11 +341,6 @@ protected:
       }
   }
 
-  void swap(DenseMapBase& RHS) {
-    std::swap(getNumEntries(), RHS.getNumEntries());
-    std::swap(getNumTombstones(), RHS.getNumTombstones());
-  }
-
   static unsigned getHashValue(const KeyT &Val) {
     return KeyInfoT::getHashValue(Val);
   }