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/foo.swift b/examples/swift/foo.swift
new file mode 100644
index 0000000..067c673
--- /dev/null
+++ b/examples/swift/foo.swift
@@ -0,0 +1,9 @@
+import class BarLib.Multiplier
+
+public class Foo {
+  public init() {}
+
+  public func multiply() -> Int {
+    return Multiplier().multiply(Constants.x, Constants.y)
+  }
+}