Fine-tune and document the barrier in TestQueues.
Fine-tune and document the barrier in TestQueues.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@350543 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 2b2983a1a65159556a1f0060a0310b40c9112aa9)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb