There is no `%swift-frontend-target` subsitution in test, which is
actually `%target-swift-frontend`. The wrong spelling is actually
interpreted by lit as `%swift`-frontend-target, and surprising didn't
break any tests as the last argument from subsitution is
`-define-availability` so it just leads to an very akward availability
definition.
`-disable-sandbox` to disable sandboxing when invoking subprocess from
from the frontend. Since `sandbox(7)` in macOS doesn't support nested
sandbox, complation used to fail when the parent build process is sandboxed.