swift234

[stdlib] On-demand unique contiguous Array storage

Lift the precondition that contiguous storage already exist on the use
of Array.withUnsafe[Mutable]PointerToElements.  Automatically guarantee
that storage is unique when using the mutable variants of these methods.

Also, fix some---essentially---inout aliasing violations in
test/stdlib/Unicode.swift that were revealed by the new careful
implementation of these methods.

Swift SVN r20339