[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
1 file changed