[libc++] ADL-proof <iterator>. __convert_to_integral is not a customization point.
The interesting change here is that we no longer consider __convert_to_integral
an ADL customization point for the user's types. I think the new behavior
is defensible. The old behavior had come from D7449, where Marshall explicitly
said "people can't define their own [__convert_to_integral overloads]."
Differential Revision: https://reviews.llvm.org/D92814