libstdc++: Workaround is_trivially_copyable<volatile T> (PR 94013)

Several algorithms check the is_trivially_copyable trait to decide
whether to dispatch to memmove or memcmp as an optimization. Since
r271435 (CWG DR 2094) the trait is true for volatile-qualified scalars,
but we can't use memmove or memcmp when the type is volatile. We need to
also check for volatile types.

This is complicated by the fact that in C++20 (but not earlier standards)
iterator_traits<volatile T*>::value_type is T, so we can't just check
whether the value_type is volatile.

The solution in this patch is to introduce new traits __memcpyable and
__memcmpable which combine into a single trait the checks for pointers,
the value types being the same, and the type being trivially copyable
but not volatile-qualified.

	PR libstdc++/94013
	* include/bits/cpp_type_traits.h (__memcpyable, __memcmpable): New
	traits to control when to use memmove and memcmp optimizations.
	(__is_nonvolatile_trivially_copyable): New helper trait.
	* include/bits/ranges_algo.h (__lexicographical_compare_fn): Do not
	use memcmp optimization with volatile data.
	* include/bits/ranges_algobase.h (__equal_fn): Use __memcmpable.
	(__copy_or_move, __copy_or_move_backward): Use __memcpyable.
	* include/bits/stl_algobase.h (__copy_move_a2): Use __memcpyable.
	(__copy_move_backward_a2): Likewise.
	(__equal_aux1): Use __memcmpable.
	(__lexicographical_compare_aux): Do not use memcmp optimization with
	volatile data.
	* testsuite/25_algorithms/copy/94013.cc: New test.
	* testsuite/25_algorithms/copy_backward/94013.cc: New test.
	* testsuite/25_algorithms/equal/94013.cc: New test.
	* testsuite/25_algorithms/fill/94013.cc: New test.
	* testsuite/25_algorithms/lexicographical_compare/94013.cc: New test.
	* testsuite/25_algorithms/move/94013.cc: New test.
	* testsuite/25_algorithms/move_backward/94013.cc: New test.

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