libstdc++: Fix three-way comparison for std::array [PR 96851]

libstdc++: Fix three-way comparison for std::array [PR 96851]

The spaceship operator for std::array uses memcmp when the
__is_byte<value_type> trait is true, but memcmp isn't usable in
constexpr contexts. Also, memcmp should only be used for unsigned byte
types, because it gives the wrong answer for signed chars with negative
values.

We can simply check std::is_constant_evaluated() so that we don't use
memcmp during constant evaluation.

To fix the problem of using memcmp for inappropriate types, this patch
adds new __is_memcmp_ordered and __is_memcmp_ordered_with traits. These
say whether using memcmp will give the right answer for ordering
operations such as lexicographical_compare and three-way comparisons.
The new traits can be used in several places.

Unlike the trunk commit this was backported from, this commit for the
branch doesn't extend the memcmp optimisations to all unsigned integers
on big endian targets. Only narrow character types and std::byte will
use memcmp.

libstdc++-v3/ChangeLog:

	PR libstdc++/96851
	* include/bits/cpp_type_traits.h (__is_memcmp_ordered):
	New trait that says if memcmp can be used for ordering.
	(__is_memcmp_ordered_with): Likewise, for two types.
	* include/bits/ranges_algo.h (__lexicographical_compare_fn):
	Use new traits instead of __is_byte and __numeric_traits.
	* include/bits/stl_algobase.h (__lexicographical_compare_aux1)
	(__is_byte_iter): Likewise.
	* include/std/array (operator<=>): Likewise. Only use memcmp
	when std::is_constant_evaluated() is false.
	* testsuite/23_containers/array/comparison_operators/96851.cc:
	New test.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.

(cherry picked from commit 2f983fa69005b603ea1758a013b4134d5b0f24a8)