Introduce an additional build product to build-script to build
back-deployable concurrency libraries. These libraries would need to
be embedded in apps deployed prior to macOS 12/iOS 15 to support
concurrency.

The built-script option `--back-deploy-concurrency` can be provided to
build these back-deployment libraries. They are built in addition to
the normal concurrency libraries, as a separate product that installs
into `lib/swift-5.5/<platform>` within the toolchain. The macro
`SWIFT_CONCURRENCY_BACK_DEPLOYMENT` is set when building the
concurrency library, so that we can adapt the implementation to older
OS's.

…yment.

When enabling the build of the back-deployable concurrency library via
the build-script option `--back-deploy-concurrency`, also build the
compiler and (main) concurrency library to support older deployment targets.

Building the compiler for older deployment targets is effectively the
same as implicitly passing
`-Xfrontend -enable-experimental-back-deploy-concurrency`. That option
should probably go away.

Building the primary _Concurrency library for back-deployment means
setting the "SwiftStdlib 5.5" availability back to the earlier
deployment targets. This should have no effect on how the _Concurrency
library binary is built, but it does ensure that the right
availability annotations are in the _Concurrency module.

…ncy.

When build-script is given `--back-deploy-concurrency`, also use that
to build other parts of Swift with the back-deployed versions:

* The compiler allows async and actors to be defined with the
back-deployed availability, e.g., the same as `-Xfrontend
-enable-experimental-back-deploy-concurrency`. (The latter will go
away soon)
* The standard library unit testing framework and distributed actors
library are build with the older OS versions.
* The tests use the older OS versions, with some adjustments to make
them agnostic to the back-deployment setting.