Fork tuple_arguments test for Swift 5.

Move the existing test into Compatibility/tuple_arguments_4.swift.
Move the Swift 3 test, Compatibility/tuple_arguments.swift, into
Compatibility/tuple_arguments_4.swift.

Add a couple new tests to each file to capture what we do in each
mode.

Overall this doesn't seem like the most satisfying approach for
covering compatibility testing in our standard commit testing, but
it's all we've got at the moment.

Author: rudkx

test/Compatibility/tuple_arguments.swift renamed to test/Compatibility/tuple_arguments_3.swift

@@ -1,7 +1,8 @@
 // RUN: %target-typecheck-verify-swift -swift-version 3
 
 // Tests for tuple argument behavior in Swift 3, which was broken.
-// The Swift 4 test is in test/Constraints/tuple_arguments.swift.
+// The Swift 4 test is in test/Compatibility/tuple_arguments_4.swift.
+// The test for the most recent version is in test/Constraints/tuple_arguments.swift.
 
 // Key:
 // - "Crashes in actual Swift 3" -- snippets which crashed in Swift 3.0.1.
@@ -1526,4 +1527,10 @@ do {
 
   func logNoOptional<T>() -> (T) -> Void { }
   f(a: logNoOptional() as ((()) -> Void)) // Also allow the optional-injected form.
+
+  func g() {}
+  g(())
+
+  func h(_: ()) {}
+  h()
 }