libstdc++/pair: Factor out const-assignability helper for C++20

This is consistent with std::tuple's __const_assignable helper, and will
be useful for implementing the new pair::operator= overloads from P2165R4.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (pair::_S_const_assignable): Define,
	factored out from ...
	(pair::operator=): ... the constraints of the const overloads.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>