swift234

[stdlib] Actually run the ArrayCore test

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