build: repair install on Windows
build: repair install on Windows

This is a result of the custom Swift handling currently in use in CMake.
CMake 3.15+ should enable proper handling of Swift and allow for correct
installation of the import library.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild