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,
instead of disabling availability checking.
An early approach to codegen for `#_hasSymbol` relied on the Darwin platfom SDK, but now that the feature lowers directly to NULL checks in LLVM IR a platform restriction is no longer needed.
However, the tests for `#_hasSymbol` remain unsupported on Windows since that OS does not support weak linking.
