Issue 9250 - Wrong error line in assignment.

http://d.puremagic.com/issues/show_bug.cgi?id=9250

Should `StringExp::implicitCastTo` also match by adding the default parameter? The compiler didn't complain, but I'm worried I might have unvirtualized this method. (I could totally use the `override` keyword in C++ right now).

Note: I could have just temporarily swapped `e2`'s loc with `this->loc`, but I think it's worthwhile having this API as it might allow future bugfixes for issues similar to this one.