Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
This patch adds a quick test to verify that we emit a reasonable error
message when trying to use async-main without enabling concurrency or
parsing as stdlib instead of crashing.
Under both circumstances, the _Concurrency library won't be implicitly
imported.
