Add an extra conversion since clocks might not necessarily return nanoseconds from time_from_epoch().

Restore original behavior that cv_status::timeout is immediately returned if the absolute time of wait_until() has already passed.
Fix return 0 location in test.

Are there any other comments for this change? I think my latest patchset takes into consideration the previous comments.

Add __safe_nanosecond_cast

I simplified the code a bit more and responded to the comments. I haven't had a good chance to test this yet, but pushing early for feedback.

Responding to review comments

So, to make sure I understand, this patch uses something that is *not yet* in POSIX, it's only proposed, correct?

Clarify that pthread_cond_clockwait() is only proposed to POSIX.
Remove enable_if.

See https://reviews.llvm.org/D65339

See https://reviews.llvm.org/D65339

Update commit message

Thanks for the comments. I'm very open to suggestions if you have any better way of doing this.

This is mostly an RFC. The underlying issue is that we cannot use timed waits based on CLOCK_REALTIME on Android. In trying to fix that, I figured I might as well try implementing the standard as described, though I understand that there'd be resistance to such a re-implementation.

I don't have commit access, so please commit for me.

Moved comments to an appropriate line