Add MapVector::rbegin(), MapVector::rend() to completment MapVector::begin(), MapVector::end().

These just delegate to the underlying vector type in the MapVector.

Also just add in some sanity unittests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220687 91177308-0d34-0410-b5e6-96231b3b80d8