Doug Gregor

@swift-ci please test

> * Didn't copy changes to `BackDeployConcurrency` yet. I expect that I'll have to make changes based on review comments, and want to apply all changes to `BackDeployConcurrency` in one...

@swift-ci please test

@swift-ci please test

@swift-ci please build toolchain macOS

I think that swiftThreading error should be gone now. I saw it late last week in an unrelated pull request, and it's gone now. However, the conflict will prevent us...

@swift-ci please smoke test and merge

@swift-ci please smoke test

@swift-ci please smoke test

@swift-ci please test source compatibility