libstdc++: Add missing feature test macros

libstdc++: Add missing feature test macros

These macros all correspond to features that are already supported, but
the macro was not defined when the feature was implemented.

	* include/bits/ptr_traits.h (__cpp_lib_constexpr_memory): Define to
	indicate P1006R1 support.
	(__cpp_lib_to_address): Define to indicate P0653R2 support.
	* include/bits/range_access.h (__cpp_lib_ssize): Define to indicate
	P1227R2 support.
	* include/bits/ranges_algo.h (__cpp_lib_shift): Define to indicate
	P0769R2 support.
	* include/std/atomic (__cpp_lib_atomic_float): Define to indicate
	P0020R6 support.
	* include/std/memory (__cpp_lib_assume_aligned): Define to indicate
	P1007R3 support.
	* include/std/memory_resource (__cpp_lib_polymorphic_allocator):
	Define to indicate P0339R6 support.
	* include/std/string_view (__cpp_lib_starts_ends_with): Define to
	indicate P0457R2 support.
	* include/std/type_traits (__cpp_lib_is_nothrow_convertible): Define
	to indicate P0758R1 support.
	(__cpp_lib_remove_cvref): Define to indicate P0550R2 support.
	(__cpp_lib_type_identity): Define to indicate P0887R1 support.
	* include/std/version (__cpp_lib_atomic_float)
	(__cpp_lib_is_nothrow_convertible, __cpp_lib_remove_cvref)
	(__cpp_lib_type_identity, __cpp_lib_assume_aligned)
	(__cpp_lib_constexpr_memory, __cpp_lib_polymorphic_allocator)
	(__cpp_lib_shift, __cpp_lib_ssize, __cpp_lib_starts_ends_with)
	(__cpp_lib_to_address): Define.
	* testsuite/20_util/to_address/1_neg.cc: Adjust dg-error line number.