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/BUILD b/examples/BUILD
index 8bfbc19..781c81a 100644
--- a/examples/BUILD
+++ b/examples/BUILD
@@ -11,5 +11,6 @@
         "//examples/py:srcs",
         "//examples/py_native:srcs",
         "//examples/shell:srcs",
+        "//examples/swift:srcs",
     ],
 )