[Commands] Cancel build when we receive a signal
[Commands] Cancel build when we receive a signal

<rdar://problem/52359638> Ctrl+C on `swift build` does not interrupt build subprocess

Looks like SwiftPM became responsible for cancelling the build when it switched to
using library-based llbuild.

abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager