libstdc++: Fix backward compatibility of P2325R3 backport [PR106320]

libstdc++: Fix backward compatibility of P2325R3 backport [PR106320]

The 11 and 10 partial backports of P2325R3, r11-9555-g92d612cccc1eec and
r10-10808-g22b86cdc4d7fdd, unnecessarily preserved some changes from the
paper that made certain view specializations no longer default
constructible, changes which aren't required to reap the overall benefits
of the paper and which are backward incompatible with pre-P2325R3 code in
practice.

This patch reverts the problematic changes, specifically it relaxes the
constraints on various views' default constructors added by the paper
so that we keep only the constraints that were already implicitly
imposed by the NSDMIs of the view.  Thus for example this patch retains
the default_initializable<_Vp> constraint on transform_view's default
constructor since its '_Vp _M_base = _Vp()' NSDMI already requires this
constraint, and it removes the default_initializable<_Fp> constraint
since the corresponding member '__detail::__box<_Fp> _M_fun' doesn't
require default constructibility (specializations of __box are always
default constructible).

After reverting these changes, all static_asserts from p2325.cc that
verify lack of default constructibility now fail as expected, matching
the pre-P2325R3 behavior.

	PR libstdc++/106320

libstdc++-v3/ChangeLog:

	* include/std/ranges (single_view): Relax constraints on
	default constructor so as to preserve pre-P2325R3 behavior.
	(filter_view): Likewise.
	(transform_view): Likewise.
	(take_while_view): Likewise.
	(drop_while_view): Likewise.
	* testsuite/std/ranges/adaptors/join.cc (test13): New test.
	* testsuite/std/ranges/p2325.cc: Fix S to be only non default
	constructible and not also non copy constructible.  XFAIL the
	tests that verify a non default constructible functor makes a
	view non default constructible (lines 94, 97 and 98).  XFAIL
	the test that effectively verifies a non default constructible
	element type makes single_view non default constructible (line
	114).