Swift concurrency defines a default platform executor. This was not
defined for FreeBSD resulting in build failures. Defining it to use the
Dispatch executor.
Scope: This only impacts FreeBSD.
Risk: Low. This change only affects FreeBSD.
Reviewers: @compnerd, @ktoso, @al45tair
Testing: Local testing to ensure Concurrency builds on FreeBSD.
Fixes: rdar://150643436
(cherry picked from commit 4a41e50730)
