[Build Script Helper] Add an option that indicates the driver is being built for use with a local just-built frontend When used, we mostly just need to be aware that the build is in a single-target mode and to simplify where the build products are placed, so that they are easier to find from the compiler's `build-script`.
[Build Script Helper] Add an option that indicates the driver is being built for use with a local just-built frontend
When used, we mostly just need to be aware that the build is in a single-target mode and to simplify where the build products are placed, so that they are easier to find from the compiler's `build-script`.

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