This implements the C++23 P1518R2 proposal "Stop overconstraining
allocators in container deduction guides" as a fix for C++17 and C++20
too.

The changes allow class template argument deduction to ignore the type
of a constructor argument that initializes an allocator_type parameter
if the type should be deducible only from the other arguments. So for
the constructor vector(const vector&, const allocator_type&) only the
first argument is used for deduction, allowing the second argument to be
anything that is implicitly convertible to argument_type. Previously
deduction would fail or an ill-formed type would be deduced if the
second argument wasn't of type allocator_type.

The unordered containers do not need changes, because their
allocator-extended constructors use the allocator_type alias, which
comes from the dependent base class so is already a non-deduced context.

libstdc++-v3/ChangeLog:

	* include/bits/forward_list.h (forward_list): Use non-deduced
	context for allocator parameter of allocator-extended copy and
	move constructors.
	* include/bits/stl_bvector.h (vector<bool>): Likewise.
	* include/bits/stl_deque.h (deque): Likewise.
	* include/bits/stl_list.h (list): Likewise.
	* include/bits/stl_map.h (map): Likewise.
	* include/bits/stl_multimap.h (multimap): Likewise.
	* include/bits/stl_multiset.h (multiset): Likewise.
	* include/bits/stl_set.h (set): Likewise.
	* include/bits/stl_vector.h (vector): Likewise.
	* include/bits/stl_queue.h (queue, priority_queue): Do not
	constrain Allocator template parameter of deduction guides that
	have a Container parameter.
	* include/bits/stl_stack.h (stack): Likewise.
	* include/debug/deque (__gnu_debug::deque): Use non-deduced
	context for allocator parameter of allocator-extended copy and
	move constructors.
	* include/debug/list (__gnu_debug::list): Likewise.
	* include/debug/map.h (__gnu_debug::map): Likewise.
	* include/debug/multimap.h (__gnu_debug::multimap): Likewise.
	* include/debug/multiset.h (__gnu_debug::multiset): Likewise.
	* include/debug/set.h (__gnu_debug::set): Likewise.
	* include/debug/vector (__gnu_debug::vector): Likewise.
	* testsuite/23_containers/deque/cons/deduction.cc: Test class
	template argument deduction with non-deduced allocator
	arguments.
	* testsuite/23_containers/forward_list/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/list/cons/deduction.cc: Likewise.
	* testsuite/23_containers/map/cons/deduction.cc: Likewise.
	* testsuite/23_containers/multimap/cons/deduction.cc: Likewise.
	* testsuite/23_containers/multiset/cons/deduction.cc: Likewise.
	* testsuite/23_containers/priority_queue/deduction.cc: Likewise.
	* testsuite/23_containers/queue/deduction.cc: Likewise.
	* testsuite/23_containers/set/cons/deduction.cc: Likewise.
	* testsuite/23_containers/stack/deduction.cc: Likewise.
	* testsuite/23_containers/unordered_map/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_multimap/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/vector/cons/deduction.cc: Likewise.

Tested powerpc64le-linux. Committed to trunk.