libstdc++: P1983R0 Wording for GB301, US296, US292, US291, and US283

Among other changes, P1983R0 resolves LWG 3278 in a different way, so this patch
also reverts the already-applied wording of LWG 3278.

The wording for US291 (the join_view::begin hunk) also required adding the
friend _Iterator<!_Const> to join_view::_Iterator.  This friend is needed so
that _Iterator's converting constructor can access the private members of an
_Iterator of the opposite constness.

The wording for US283 has already been applied it seems.

libstdc++-v3/ChangeLog:

	P1983R0 Wording for GB301, US296, US292, US291, and US283
	* include/std/ranges (filter_view::pred): New member function.
	(join_view::_Iterator::_Iterator): Remove now-redundant comment since
	P1983R0 fixes the highlighted issue in the same way.
	(join_view::_Iterator<_Const>): Add friend
	join_view::_Iterator<!_Const>.
	(join_view::_M_inner): Remove mutable specifier, effectively reverting
	the proposed wording changes of P3278.
	(join_view::begin): Refine the condition for when to return a const
	iterator.
	(split_view::_OuterIter::_OuterIter): Adjust constraints.
	* testsuite/std/ranges/adaptors/filter.cc: Test that filter_view::pred
	exists and works.