libstdc++: Various fixes for atomic wait/notify code

libstdc++: Various fixes for atomic wait/notify code

Pass __wait_args_base by const reference instead of const pointer. I
don't see a reason it needs to be passed by pointer to the internals.
We can also avoid constructing a __wait_args from __wait_args_base in
some places, instaad just using the latter directly.

The code using the __wait_flags bitmask type is broken, because the
__spin_only constant includes the __do_spin element. This means that
testing (__args & __wait_flags::__spin_only) will be inadvertently true
when only __do_spin is set. This causes the __wait_until_impl function
to never actually wait on the futex (or condition variable), turning all
uses of that function into expensive busy spins. Change __spin_only to
be a single bit (i.e. a bitmask element) and adjust the places where
that bit is set so that they also use the __do_spin element.

Update the __args._M_old value when looping in __atomic_wait_address, so
that the next wait doesn't fail spuriously.

With the new __atomic_wait_address logic, the value function needs to
return the correct type, not just a bool. Without that change, the
boolean value returned by the value function is used as the value
passed to the futex wait, but that mean we're comparing (_M_a == 0) to
_M_a and so can block on the futex when we shouldn't, and then never
wake up.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__cond_wait_impl): Add
	missing inline keyword.
	(__spin_until_impl): Change parameter from pointer to reference.
	Replace make_pair with list-initialization.  Initialize variable
	for return value.
	(__wait_until_impl): Likewise. Remove some preprocessor
	conditional logic. Use _S_track for contention tracking.
	Avoid unnecessary const_cast.
	(__wait_until): Change parameter from pointer to reference.
	Replace make_pair with list-initialization.
	(__wait_for):  Change parameter from pointer to reference. Add
	__do_spin flag to args.
	* include/bits/atomic_wait.h (__waiter_pool_impl::_S_track): New
	function returning an RAII object for contention tracking.
	(__wait_flags): Do not set the __do_spin flag in the __spin_only
	enumerator. Comment out the unused __abi_version_mask
	enumerator.  Define operator| and operator|= overloads.
	(__wait_args_base::operator&): Define.
	(__wait_args::operator&, __wait_args::_S_default_flags): Remove.
	(__wait_args::operator|, __wait_args::operator|=): Remove.
	(__spin_impl): Change parameter from pointer to reference.
	Replace make_pair call with list-initialization.
	(__wait_impl): Likewise.  Remove some preprocessor conditional
	logic.  Always store old value in __args._M_old. Avoid
	unnecessary const_cast. Use _S_track.
	(__notify_impl): Change parameter to reference. Remove some
	preprocessor conditional logic.
	(__atomic_wait_address): Add comment. Update __args._M_old on
	each iteration.
	(__atomic_wait_address_v): Add comment.
	* include/std/latch (latch::wait): Adjust predicates for new
	logic.
	* testsuite/29_atomics/atomic_integral/wait_notify.cc: Improve
	test.