[libc++] Move __libcpp_erase_if_container into <iterator>, and ADL-proof it.

The container headers don't need to include <functional> for any other reason
(or at least, they wouldn't if we moved `less` and `equal_to` out of <functional>),
so let's put `__libcpp_erase_if_container` somewhere that's common to the
containers but outside of <functional>.

Also, calling `std::erase_if(c, pred)` should not trigger ADL.

Differential Revision: https://reviews.llvm.org/D99043