IR: Expose Module::rbegin() and rend()

A follow-up commit for PR5680 needs to visit functions in reverse order.
Expose iterators to allow that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214121 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed