PR17666: Instead of allowing an initial identifier argument in any attribute
which we don't think can't have one, only allow it in the tiny number of
attributes which opts into this weird parse rule.

I've manually checked that the handlers for all these attributes can in fact
cope with an identifier as the argument. This is still somewhat terrible; we
should move more fully towards picking the parsing rules based on the
attribute, and make the Parse -> Sema interface more type-safe.