More pleasant to work with than ObjectiveC, which will help with the
following commit.
Swift libraries weren't added to macOS until 10.14.4, so theme
autodetection will fail on 10.14.0-10.14.3. The Qt6 build will have its
minimum version bumped to 10.14.4; the Qt5 build will remain on 10.13.4.
Bazel's rules_swift doesn't currently support building Swift dylibs, so
we need to invoke swiftc directly via a genrule().
