libstdc++: Fixes for feature test macros (PR 91480)

libstdc++: Fixes for feature test macros (PR 91480)

Remove the non-standard __cpp_lib_allocator_is_always_equal macro and
add the missing macros for P1032R1.

	PR libstdc++/91480
	* include/bits/allocator.h (__cpp_lib_allocator_is_always_equal):
	Remove non-standard macro.
	* include/bits/stl_iterator.h (__cpp_lib_constexpr_iterator): Define
	to indicate P1032R1 support.
	* include/bits/stl_pair.h (__cpp_lib_constexpr_utility): Likewise.
	* include/std/string_view (__cpp_lib_constexpr_string_view): Likewise.
	* include/std/tuple (__cpp_lib_constexpr_tuple): Likewise.
	* include/std/version (__cpp_lib_allocator_is_always_equal): Remove.
	(__cpp_lib_constexpr_iterator, __cpp_lib_constexpr_string_view)
	(__cpp_lib_constexpr_tuple, __cpp_lib_constexpr_utility): Define.
	* testsuite/20_util/function_objects/constexpr_searcher.cc: Check
	feature test macro.
	* testsuite/20_util/tuple/cons/constexpr_allocator_arg_t.cc: Likewise.
	* testsuite/21_strings/basic_string_view/operations/copy/char/
	constexpr.cc: Likewise.
	* testsuite/24_iterators/insert_iterator/constexpr.cc: Likewise.