[PlaygroundTransform] Instrument mutations of inout vars by class methods.

+= is now a class method, which means the playground transform ignores it.  This
is undesirable, and we have ways to instrument methods that mutate inout
parameters the way += does.  I have just made it so that if we see a method call
and we can't instrument the mutation of the base of the method call, we still
instrument the inout argument if there is one.

Also added a test case.

Author: scallanan

lib/Sema/PlaygroundTransform.cpp

@@ -611,15 +611,19 @@ class Instrumenter {
                   Elements.insert(Elements.begin() + (EI + 1), *Log);
                   ++EI;
                 }
+                Handled = true;
               }
-            } else if (DeclRefExpr *DRE = digForInoutDeclRef(AE->getArg())) {
-              Added<Stmt *> Log = logDeclOrMemberRef(DRE);
-              if (*Log) {
-                Elements.insert(Elements.begin() + (EI + 1), *Log);
-                ++EI;
+            }
+            if (!Handled) {
+              if (DeclRefExpr *DRE = digForInoutDeclRef(AE->getArg())) {
+                Added<Stmt *> Log = logDeclOrMemberRef(DRE);
+                if (*Log) {
+                  Elements.insert(Elements.begin() + (EI + 1), *Log);
+                  ++EI;
+                }
+                Handled = true;
               }
             }
-            Handled = true;
           }
           if (!Handled) {
             // do the same as for all other expressions

test/PlaygroundTransform/plus_equals.swift

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: 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
+
+var a = "a"
+var b = "b" 
+a += b 
+// CHECK: [{{.*}}] $builtin_log[a='a']
+// CHECK-NEXT: [{{.*}}] $builtin_log[b='b']
+// CHECK-NEXT: [{{.*}}] $builtin_log[='ab']