swift 2.15.1 release

meta:version: 2.15.1
meta:diff-start: -
meta:series: pike
meta:release-type: release
meta:pypi: no
meta:first: no
meta:release:Author: John Dickinson <me@not.mn>
meta:release:Commit: John Dickinson <me@not.mn>
meta:release:Change-Id: Id33f03c3113ba8f99f03a7902a763d1f186cc0d8
meta:release:Code-Review+2: Sean McGinnis <sean.mcginnis@gmail.com>
meta:release:Code-Review+2: Davanum Srinivas (dims) <davanum@gmail.com>
meta:release:Workflow+1: Davanum Srinivas (dims) <davanum@gmail.com>
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