swift234

[stdlib] Don't try to swap an element with itself in 'partition'.

(or in 'randomShuffle')

This violates our inout aliasing rules, and can cause a crash in certain
situations.

To try to prevent this in the future I've added a debugPrecondition check
to 'swap' that checks addresses. This will only catch inout violations
where neither argument uses writeback, and might some day be optimized
away. Right now, though, it does seem to catch some simple violations
(though not the one in the Radar), so hopefully it will at least help keep
us from introducing new problems.

rdar://problem/21780567

Swift SVN r30956