iterator_range: Add an llvm::make_range() helper method.

Convenience wrapper to make dealing with sub-ranges easier. Like the
iterator_range<> itself, if/when this sort of thing gets standards
blessing, it will be replaced by the official version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205987 91177308-0d34-0410-b5e6-96231b3b80d8