This patch is the first part of library support for constexpr
std::vector and std::string. This only includes the changes to
std::allocator, std::allocator_traits, std::construct_at,
std::destroy_at, std::destroy and std::destroy_n.

std::allocator::allocate and std::allocator::deallocate need to be
added so that they can be intercepted by the compiler during constant
evaluation. Outside of constant evaluation those new member functions
just forward to the existing implementation in the base class.

	PR c++/91369 Implement P0784R7 changes to allocation and construction
	* include/bits/alloc_traits.h: Include <bits/stl_construct.h>.
	(allocator_traits::_S_allocate, allocator_traits::_S_construct)
	(allocator_traits::_S_destroy, allocator_traits::_S_max_size)
	(allocator_traits::_S_select, allocator_traits::allocate)
	(allocator_traits::deallocate, allocator_traits::construct)
	(allocator_traits::destroy, allocator_traits::max_size)
	(allocator_traits::select_on_container_copy_construction)
	(allocator_traits<allocator<T>>): Add constexpr specifier for C++20.
	(allocator_traits<allocator<T>>::construct): Use construct_at.
	(allocator_traits<allocator<T>>::destroy): Use destroy_at.
	(__alloc_on_copy, __alloc_on_move, __alloc_on_swap): Add constexpr
	specifier.
	(_Destroy(ForwardIterator, ForwardIterator, Alloc&))
	(_Destroy(ForwardIterator, ForwardIterator, allocator<T>&)): Move here
	from <bits/stl_construct.h>.
	* include/bits/allocator.h (allocator::~allocator): Remove for C++20.
	(allocator::allocate, allocate::deallocate): Define for C++20 and up.
	(operator==, operator!=): Add constexpr specifier for C++20.
	* include/bits/stl_construct.h: Don't include <ext/alloc_traits.h>.
	(destroy_at): For C++20 add constexpr specifier and support for
	destroying arrays.
	(construct_at): Define new function for C++20.
	(_Construct): Return result of placement new-expression. For C++11 and
	up add constexpr. For C++20 dispatch to std::construct_at during
	constant evaluation.
	(_Destroy(pointer)): Add constexpr specifier. For C++20 dispatch to
	std::destroy_at during constant evaluation.
	(_Destroy_aux::__destroy, _Destroy_n_aux::__destroy_n): Add constexpr
	specifier for C++20.
	(_Destroy(ForwardIterator, ForwardIterator))
	(_Destroy(ForwardIterator, Size)): Likewise. Do not elide trivial
	destructors during constant evaluation.
	(destroy, destroy_n): Add constexpr specifier for C++20.
	(_Destroy(ForwardIterator, ForwardIterator, Alloc&))
	(_Destroy(ForwardIterator, ForwardIterator, allocator<T>&)): Move to
	<bits/alloc_traits.h>, to remove dependency on allocators.
	* include/bits/stl_uninitialized.h: Include <ext/alloc_traits.h>.
	Include <bits/stl_pair.h> instead of <utility>.
	* include/ext/alloc_traits.h: Always include <bits/alloc_traits.h>.
	(__alloc_traits::construct, __alloc_traits::destroy)
	(__alloc_traits::_S_select_on_copy, __alloc_traits::_S_on_swap): Add
	constexpr specifier.
	* include/ext/malloc_allocator.h  (operator==, operator!=): Add
	constexpr specifier for C++20.
	* include/ext/new_allocator.h (operator==, operator!=): Likewise.
	* testsuite/20_util/headers/memory/synopsis.cc: Add constexpr.
	* testsuite/20_util/scoped_allocator/69293_neg.cc: Ignore additional
	errors due to constexpr function called after failed static_assert.
	* testsuite/20_util/specialized_algorithms/construct_at/1.cc: New test.
	* testsuite/23_containers/vector/cons/destructible_debug_neg.cc:
	Ignore additional errors due to constexpr function called after failed
	static_assert.
	* testsuite/23_containers/vector/cons/destructible_neg.cc: Likewise.

From-SVN: r277342