Modify SizeofExpr argument

[a3moss]

Currently, SizeofExpr has both type and expr members, which are mutually exclusive. It may be a worthwhile simplification to just use the expr member, with TypeExpr representing a type.

The motivating case would be allowing attributes on the argument of a sizeof expression, e.g:

sizeof(int __attribute__ ((aligned (8))))

Currently this is handled by allowing all types (not just ReferenceToType) to have a list of attributes, but the attribute list could be moved back down to ReferenceToType in this case.