Add a Skylark rule to build Swift modules.

* Adds a swift_library rule that produces a (.a, .swiftmodule) pair. It can handle dependencies between modules and can be used as a dependency of objc_binary.
* Does not work with Objective-C yet.

--
MOS_MIGRATED_REVID=120578875
diff --git a/examples/swift/constants.swift b/examples/swift/constants.swift
new file mode 100644
index 0000000..6c16d34
--- /dev/null
+++ b/examples/swift/constants.swift
@@ -0,0 +1,4 @@
+class Constants {
+  static var x = 2
+  static var y = 3
+}
\ No newline at end of file