C++0x introduces a new taxonomy for expressions. We've got lvalues,
xvalues and prvalues, and their groupings glvalues and rvalues. These
are mostly relevant for reference binding, but that also includes casts.
I wonder, would it be better to try to integrate classification with the
current Expr::isLvalue function (e.g. extend the LV_Result enum with
LV_XValue), or should I write a separate function for that?