[CodeCompletion] Add assignment to experimental operator completion

When the LHS is an lvalue/assignable tuple and there is no leading
sequence of binary expressions.

It's a bit hacky right now since we don't have a good way to
differentiate general pattern completions from builtin operators.

rdar://problem/23209683

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -263,6 +263,9 @@ static void getResultStructure(
         C.is(ChunkKind::CallParameterBegin))
       break;
 
+    if (C.is(ChunkKind::Equal))
+      isOperator = true;
+
     if (C.hasText())
       textSize += C.getText().size();
   }