libstdc++: Define __wait_result_type for atomic waiting

libstdc++: Define __wait_result_type for atomic waiting

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h: Use __wait_result_type.
	* include/bits/atomic_wait.h (__wait_result_type): New struct.
	(__wait_args::_M_prep_for_wait_on): Rename to _M_setup_wait, use
	__wait_result_type.
	(__atomic_wait_address): Adjust to call _M_setup_wait.
	* src/c++20/atomic.cc (__spin_impl): Use __wait_result_type.
	(__wait_impl): Likewise.
	(__spin_until_impl): Likewise.
	(__wait_until_impl): Likewise.