libstdc++: add support for constexpr stable_sort (P2562R1)

libstdc++: add support for constexpr stable_sort (P2562R1)

stable_sort has been made constexpr in C++26. Apart from plastering a
few functions with constexpr, there's an implementation challenge, that
is: stable_sort takes different codepaths in case extra memory can be
allocated. Rather than doing some major refactorings, simply use the
non-allocating path during constant evaluation. That's the same codepath
used when extra memory could not be allocated, as well as by
freestanding.

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h (stable_sort): Add constexpr.
	* include/bits/ranges_algo.h (__stable_sort_fn): Add constexpr
	to the function call operators.
	* include/bits/stl_algo.h (__stable_sort): Add constexpr.
	During constant evaluation, always use the non-allocating path.
	(stable_sort): Add constexpr.
	(__inplace_stable_sort): Likewise.
	(__merge_without_buffer): Likewise.
	* include/bits/version.def (constexpr_algorithms): Bump value
	for C++26.
	* include/bits/version.h: Regnerate.
	* testsuite/25_algorithms/cpp_lib_constexpr.cc: Test the bumped
	feature-testing macro.
	* testsuite/25_algorithms/headers/algorithm/synopsis.cc: Adapt
	the test to constexpr stable_sort.
	* testsuite/25_algorithms/stable_sort/constexpr.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>