gen_log: prints parcelable/union/enum types

User defined types are printed using toString().

Bug: 161439795
Test: aidl_integration_test
Change-Id: I10fbac5f268d19791899129e339dd7c64638be99
diff --git a/aidl_to_cpp_common.h b/aidl_to_cpp_common.h
index d11b067..2b2db4a 100644
--- a/aidl_to_cpp_common.h
+++ b/aidl_to_cpp_common.h
@@ -29,6 +29,9 @@
 namespace aidl {
 namespace cpp {
 
+// provides _call_toString(expr) which call expr.toString() if possible
+extern char kToStringHelper[];
+
 // These roughly correspond to the various class names in the C++ hierarchy:
 enum class ClassNames {
   BASE,          // Foo (not a real class, but useful in some circumstances).