swift 2.20.0 release

meta:version: 2.20.0
meta:diff-start: -
meta:series: stein
meta:release-type: release
meta:pypi: no
meta:first: yes
meta:release:Author: John Dickinson <me@not.mn>
meta:release:Commit: John Dickinson <me@not.mn>
meta:release:Change-Id: I1d3f4d29376eddde6aaf9b52f3361c5909d283b7
meta:release:Code-Review+1: Hervé Beraud <hberaud@redhat.com>
meta:release:Code-Review+1: Jeremy Stanley <fungi@yuggoth.org>
meta:release:Code-Review+1: Kendall Nelson <kennelson11@gmail.com>
meta:release:Code-Review+1: foundjem <foundjem@ieee.org>
meta:release:Code-Review+2: Doug Hellmann <doug@doughellmann.com>
meta:release:Workflow+1: Doug Hellmann <doug@doughellmann.com>
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