[PlaygroundTransform] Don't log the results of functions that return ().

The fix for missing logging for += accidentally made us log ALL functions that
return (), not just the ones that happen to touch inout parameters.  That's not
really desirable, and resulted from a missing testcase.

This fixes the problem and adds a testcase.

<rdar://problem/27995558>

Author: scallanan

lib/Sema/PlaygroundTransform.cpp

@@ -621,9 +621,9 @@ class Instrumenter {
                   Elements.insert(Elements.begin() + (EI + 1), *Log);
                   ++EI;
                 }
-                Handled = true;
               }
             }
+            Handled = true; // Never log ()
           }
           if (!Handled) {
             // do the same as for all other expressions

test/PlaygroundTransform/empty-tuple.swift

@@ -0,0 +1,9 @@
+// RUN: rm -rf %t && mkdir %t
+// RUN: cp %s %t/main.swift
+// RUN: %target-build-swift -Xfrontend -playground -Xfrontend -debugger-support -o %t/main %S/Inputs/PlaygroundsRuntime.swift %t/main.swift
+// RUN: %target-run %t/main | %FileCheck %s
+// REQUIRES: executable_test
+func foo() { }
+foo()
+1+2
+// CHECK: {{.*}} $builtin_log[='3']