libstdc++: Fix time_point conversion in atomic timed waits

libstdc++: Fix time_point conversion in atomic timed waits

Even if a time_point already uses the right clock, we might still need
to convert it to use the expected duration. Calling __to_wait_clock will
perform that conversion, so use that even when the clock is correct.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__to_wait_clock): Do not use
	chrono::ceil if clock and duration are already correct type.
	(__wait_until): Always call __to_wait_clock.