[libcxx] Fix enable_if condition of std::reverse_iterator::operator=

The template std::is_assignable<T, U> checks that T is assignable from
U. Hence, the order of operands in the instantiation of
std::is_assignable in the std::reverse_iterator::operator= condition
should be reversed.

This issue remained unnoticed because std::reverse_iterator has an
implicit conversion constructor. This patch adds a test to check that
the assignment operator is used directly, without any implicit
conversions. The patch also adds a similar test for
std::move_iterator.

Reviewed By: Quuxplusone, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D113417