Fix warnings in Bazel Swift example library

* Require callers to use argument labels so that the call site looks the same in Swift 2 and 3.

--
PiperOrigin-RevId: 144484042
MOS_MIGRATED_REVID=144484042

examples/swift/BarLib/mul.swift

diff --git a/examples/swift/BarLib/mul.swift b/examples/swift/BarLib/mul.swift
index 64d66cd..898c0b6 100644
--- a/examples/swift/BarLib/mul.swift
+++ b/examples/swift/BarLib/mul.swift
@@ -2,7 +2,7 @@
 public class Multiplier {
   public init() {}
 
-  public func multiply(_ a: Int, _ b: Int) -> Int {
-      return a * b
+  public func multiply(a first: Int, b second: Int) -> Int {
+      return first * second
   }
 }

examples/swift/foo.swift

diff --git a/examples/swift/foo.swift b/examples/swift/foo.swift
index 1dd35c4..4f80dad 100644
--- a/examples/swift/foo.swift
+++ b/examples/swift/foo.swift
@@ -4,6 +4,6 @@
   public init() {}
 
   public func multiply() -> Int {
-    return Multiplier().multiply(Constants.x, Constants.y)
+    return Multiplier().multiply(a: Constants.x, b: Constants.y)
   }
 }