[concurrency] Implement a compatibility .a library for Concurrency.

[DougGregor]

In a back deployment scenario, this will provide a place where one could provide
function implementations that are not available in the relevant stdlib.

This is just setting up for future work and isn't doing anything interesting
beyond wiring it up/making sure that it is wired up correctly with tests.

[DougGregor]

@swift-ci please build toolchain

[DougGregor]

@swift-ci please build toolchain Linux

[swift-ci]

Linux Toolchain (Ubuntu 16.04)
Download Toolchain
Git Sha - 1bc94bf

Install command
tar zxf swift-PR-38930-643-ubuntu16.04.tar.gz
More info

[DougGregor]

swiftlang/swift-package-manager#3695

@swift-ci please build toolchain

[DougGregor]

swiftlang/swift-package-manager#3695

@swift-ci please test

[DougGregor]

Please test with the following pull request:
swiftlang/swift-package-manager#3695

@swift-ci please build toolchain

[swift-ci]

Linux Toolchain (Ubuntu 16.04)
Download Toolchain
Git Sha - 1bc94bf

Install command
tar zxf swift-PR-38930-665-ubuntu16.04.tar.gz
More info

[DougGregor]

Closer!

[DougGregor]

We got much further this time. Now it's a playgrounds logger issue that looks potentially spurious?

07:29:11 Testing failed:
07:29:11 	PlaygroundLoggerTests_macOS:
07:29:11 		The test runner encountered an error (Failed to establish communication with the test runner. If you believe this error represents a bug, please attach the result bundle at /Users/buildnode/jenkins/workspace/swift-PR-toolchain-osx/branch-main/buildbot_osx/playgroundsupport-macosx-x86_64/DerivedData/Logs/Test/Test-BuildScript-Test-PlaygroundLogger-macOS-2021.08.28_05-28-47--0700.xcresult. (Underlying Error: Couldn’t communicate with a helper application. Try your operation again. If that fails, quit and relaunch the application and try again. The connection to service on pid 0 named com.apple.testmanagerd.control was invalidated.))

[DougGregor]

Please test with the following pull request:
swiftlang/swift-package-manager#3695

@swift-ci please build toolchain macOS

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - 1bc94bf

Install command
tar -zxf swift-PR-38930-1110-osx.tar.gz --directory ~/

[DougGregor]

Success!

[DougGregor]

swiftlang/swift-package-manager#3695 has now been merged, so this can be merged as well