Issue Description:

SwiftPM has an option to emit the .swiftmodule for a particular target as a separate -emit-module action, rather than generating partial .swiftmodules and then calling invoking the merge-modules phase. SwiftPM does so by forming two different invocations of the Swift driver, which is suboptimal both because it puts a lot of this logic directly into SwiftPM and because there is extra overhead in having two distinct Swift driver instances for the same task.

We should implement --emit-swift-module-separately in the new Swift driver, then teach SwiftPM to pass --emit-swift-module-separately into the integrated Swift driver. This centralizes the logic for --emit-swift-module-separately and lets the new Swift driver implement the proper dependency graph.