libstdc++: Provide conversion between atomic_ref of similar types.

libstdc++: Provide conversion between atomic_ref of similar types.

This patch implements the P3860R1 (accepted as DR against C++20) and LWG4472.

The two constraints on the constructor (that T and U are similar types and
is_convertible_v<U*, T*>) are combined into a single check:
is_convertible_v<_Up(*)[1], _Tp(*)[1]>. While this check is not equivalent
for array of known bound to array of unknown bound conversions (T[N] to T[]),
this is irrelevant for atomic_ref, since instantiation with an array type is
ill-formed (due to the return type of load and other members).

The __atomic_ref_base constructor is modified to accept _Tp* instead of _Tp&.
This allows both the atomic_ref(atomic_ref<_Up> __other) and atomic_ref(_Tp& __t)
constructors to delegate to it. Furthermore, such approach does not require
dereferencing *__other._M_ptr, and thus avoid ADL-lookup for operator* and
issues related to it. The precondition check on alignment is moved specifically
to the atomic_ref(_Tp&) constructor, preventing redundant checks during atomic_ref
conversion.

A deleted atomic_ref(_Tp&&) constructor is introduced (per LWG4472).
This prevents the construction of atomic_ref<T> from atomic_ref<volatile T>
via the conversion operator.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_base.h
	(__atomic_ref_base<const _Tp>::__atomic_ref_base): Accept
	pointer instead of reference. Remove precondition check and
	mark as noexcept.
	(__atomic_ref_base<_Tp>::__atomic_ref_base): Accept pointer
	insted of reference, and mark as noexcept.
	* include/std/atomic (atomic_ref::atomic_ref(_Tp&)): Add
	precondition check and take address of argument.
	(atomic_ref::atomic_ref(_Tp&&)): Define as deleted.
	(atomic_ref::atomic_ref(atomic_ref<_Up>)): Define.
	* include/bits/shared_ptr_atomic.h (_Sp_atomic::_Atomic_count):
	Pass address to __atomic_ref constructor.
	* include/std/barrier (__tree_barrier_base::_M_arrive)
	(__tree_barrier::arrive): Pass address to __atomic_ref constructor.
	* testsuite/29_atomics/atomic_ref/ctor.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>