[CodeCompletion] Don't complete operator = after Void result

print(1) = blah

doesn't make much sense.

lib/IDE/CodeCompletion.cpp

@@ -3087,7 +3087,7 @@ public:
     }
 
     if (leadingSequence.empty() && LHS->getType() &&
-        LHS->getType()->isAssignableType()) {
+        LHS->getType()->isLValueType()) {
       addAssignmentOperator(LHS->getType()->getRValueType(),
                             CurrDeclContext->getASTContext().TheEmptyTupleType);
     }