[bootstrap] Build llbuild as part of bootstrap
[bootstrap] Build llbuild as part of bootstrap

This will build llbuild automatically as part of the bootstrap
invocation if the --llbuild-build-dir option is not passed.

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