Implement LWG685 (which is from C++11!). Fixes PR#28421. Note: this (subtly)...

Implement LWG685 (which is from C++11!). Fixes PR#28421.  Note: this (subtly) changes the return type of operator-(Iter1, Iter2) where Iter1 is a reverse iterator or a move_iterator, and Iter2 is some other move/reverse iterator type. In practice, I believe that almost every time the second param will be const_XXX and this will mean that the return type will be the same as it was before.

llvm-svn: 274880