swift

73cfe346 - Allow -explicit-swift-module-map-file without full explicit module build

Allow -explicit-swift-module-map-file without full explicit module build This enables the use of `-explicit-swift-module-map-file` for some modules in the build, while still loading implicit modules as before. This is useful to improve the performance of builds with many modules to avoid searching many different directories pass with `-I`. Previously VFS overlays could be used for this use case as well, but it seems valuable to unify on the same infrastructure used for explicit module builds.