swift234

[stdlib] Remove _buffer requirement from ArrayType

Until we get an optimizer pass to remove get/set pairs, passing a
property that is a protocol requirement as inout from generic code is
always going to cause an extra retain, causing many unintended Array
copies.

Because this dropped reference counts to 1 in some cases, it exercised
previously-untested code paths and uncovered bugs, particularly in the
handling of subrange replacement on Slice<T>.

There are still differences in speed for short arrays of CGPoint that bear
investigation, but at least as things scale up, the ratio of time goes
to 1.

Fixes <rdar://problem/17040913> append and += on an array have
completely different performance

Swift SVN r19228