Add DenseMap::insert(value_type&&) method.
Use the existing move implementation of the internal DenseMap::InsertIntoBucket
method to provide a user-facing move insert method.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172453 91177308-0d34-0410-b5e6-96231b3b80d8