SwiftBuild should query SwiftDriver instead of executing swiftc --version for Swift compiler version information when using the integrated driver library

[cwakamo]

SwiftBuild currently executes swiftc --version to get Swift compiler version information early in its execution. However, SwiftDriver can provide this information as well (it has access to it from libSwiftScan's swiftscan_compiler_target_info_query_v2 function). To minimize process spawns, and to avoid needing to use the swiftc binary when it otherwise wouldn't be used, SwiftBuild should query SwiftDriver for this information when the integrated driver library is in use.

[cwakamo]

It looks like SwiftDriver has this information, but doesn't expose it in its API surface. Assuming we wanted to do this in SwiftBuild, I don't think it would be unreasonable to add an API to SwiftDriver to get this information.