Fix "use after move" in iterator_range

A static analysis tool reported a "use after move" error in a place where the
same object was being passed to two different calls using std::forward. While
this seems unlikely to present an actual problem, removing the std::forward
makes the code slightly more robust.

This relates to the comments here: https://reviews.llvm.org/D152891#inline-1522454

Differential Revision: https://reviews.llvm.org/D157453