libstdc++: Add freestanding feature test macros (P2407R5)

libstdc++: Add freestanding feature test macros (P2407R5)

This C++26 change makes several classes "partially freestanding", but we
already fully supported them in freestanding mode. All we need to do is
define the new feature test macros and add tests for them.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (freestanding_algorithm)
	(freestanding_array, freestanding_optional)
	(freestanding_string_view, freestanding_variant): Add.
	* include/bits/version.h: Regenerate.
	* include/std/algorithm (__glibcxx_want_freestanding_algorithm):
	Define.
	* include/std/array (__glibcxx_want_freestanding_array):
	Define.
	* include/std/optional (__glibcxx_want_freestanding_optional):
	Define.
	* include/std/string_view
	(__glibcxx_want_freestanding_string_view): Define.
	* include/std/variant (__glibcxx_want_freestanding_variant):
	Define.
	* testsuite/20_util/optional/version.cc: Add checks for
	__cpp_lib_freestanding_optional.
	* testsuite/20_util/variant/version.cc: Add checks for
	__cpp_lib_freestanding_variant.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.
	* testsuite/21_strings/basic_string_view/requirements/version.cc:
	New test.
	* testsuite/23_containers/array/requirements/version.cc: New
	test.
	* testsuite/25_algorithms/fill_n/requirements/version.cc: New
	test.
	* testsuite/25_algorithms/swap_ranges/requirements/version.cc:
	New test.