libstdc++: Use allocate_at_least in vector, string (P0401) [PR118030]

libstdc++: Use allocate_at_least in vector, string (P0401) [PR118030]

Implement as much of allocator<>::allocate_at_least as possible
relying solely on known alignment behavior of standard operator
new.

Use allocator_at_least in string and vector to maximize usage of
actually allocated storage, as revealed by the allocator in use.
For user-supplied allocators this may make a big difference.

Nothing is changed in include/ext/malloc_allocator or others.
They can be updated at leisure, piecemeal.

libstdc++-v3/ChangeLog:
	PR libstdc++/118030
	* config/abi/pre/gnu.ver: Expose string::_S_allocate_at_least,
	_M_create_plus symbols.
	* include/bits/alloc_traits.h:
	(allocate_at_least): Delegate in allocator_traits<allocator<_Tp>>
	specialization to allocator<_Tp>::allocate_at_least, unconditionally;
	annotate [[__gnu__::always_inline__]].
	(allocate_at_least): Declare "= delete;" in allocator<void>.
	* include/bits/allocator.h (allocate_at_least): Delegate to base
	allocate_at_least where defined, calling with explicit base-class
	qualification, picking up __new_allocator member.
	* include/bits/basic_string.h:
	(_Alloc_result): Define new type.
	(_S_allocate_at_least): Define, using it.
	(_S_allocate): Minimize for legacy ABI use only.
	(_M_create_plus): Declare.
	(_M_create_and_place): Define, abstracting common operations.
	(assign): Use _S_allocate_at_least.
	* include/bits/basic_string.tcc:
	(_M_create_plus): Define.
	(_M_replace, reserve): Use _S_allocate_at_least.
	(_M_construct, others (3x)): Use _M_create_and_place.
	(_M_construct, input iterators): Use _M_create_plus.
	(_M_create, _M_assign, reserve, _M_mutate): Same.
	* include/bits/memory_resource.h (allocate_at_least): Define,
	document.
	* include/bits/new_allocator.h (allocate_at_least): Define.
	(_S_check_allocation_limit) Define.
	(allocate): Use _S_check_allocation_limit.
	(_S_max_size): Change from _M_max_size.
	(deallocate): Refine "if constexpr" logic.
	* include/bits/stl_vector.h:
	(_S_max_size): Move to _Vector_base.
	(_Alloc_result): Define type.
	(_M_allocate_at_least): Define, using allocate_at_least where supported.
	(_M_allocate): Delegate to _M_allocate_at_least.
	(max_size, _S_check_init_len): Use _S_max_size as moved.
	(_M_create_storage, append_range, _M_allocate_and_copy,
	_M_replace_storage): Define, abstracting common operations.
	(_M_replace_with): Define, likewise.
	(_M_range_initialize_n): Use _M_allocate_at_least.
	(_M_check_len): Improve logic.
	* include/bits/vector.tcc:
	(reserve, _M_fill_append, _M_range_insert): Use _M_allocate_at_least
	and _M_replace_storage.
	(operator=, _M_assign_aux): Use _M_replace_with.
	(_M_realloc_insert, _M_realloc_append, _M_default_append, insert_range):
	Use _M_allocate_at_least.
	(_M_fill_insert): Use _M_replace_storage, normalize whitespace.
	* testsuite/util/testsuite_allocator.h:
	(allocate_at_least (3x)): Define.
	(allocate): Use allocate_at_least.
	* testsuite/20_util/allocator/allocate_at_least.cc: Add tests.
	* testsuite/21_strings/basic_string/capacity/char/18654.cc:
	Loosen capacity check.
	* testsuite/21_strings/basic_string/capacity/char/shrink_to_fit.cc:
	Same.
	* testsuite/21_strings/basic_string/capacity/wchar_t/18654.cc: Same.
	* testsuite/21_strings/basic_string/capacity/wchar_t/2.cc: Same.
	* testsuite/21_strings/basic_string/capacity/wchar_t/shrink_to_fit.cc:
	Same.
	* testsuite/23_containers/vector/capacity/shrink_to_fit.cc: Same.
	* testsuite/23_containers/vector/capacity/shrink_to_fit2.cc: Same
	* testsuite/23_containers/vector/modifiers/emplace/self_emplace.cc:
	Adapt to looser reserve behavior.