Fixes rdar://problem/18646425
Running the test uncovered avoidable inefficiencies in Array copying, so
dispatch of _copyToNativeArrayBuffer was revamped. It's reasonable to
expect some speedups from this.
Also, the internal Array API requestNativeBuffer was highly error prone
when the source was a Slice, because it could return an array buffer
that represented more than the entire slice. That capability was
probably used for optimization once, but is no longer, and the error
prone API probably caused bugs, so it was reformed.
Swift SVN r22746
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745
