[Build] Add a helper for integration into Swift's build-script.
[Build] Add a helper for integration into Swift's build-script.

Allows building, testing, and installing swift-driver and swift-help
from Swift's build-script. Note that the installed swift-driver does
not yet replace `swift` or `swiftc`.

apple/swift-driver,apple/swift-driver,apple/swift-driver,apple/swift-driver