Move away from 'general' / 'generalized' as a way of identifying C++11
attribute syntax. There's nothing generalized about this; it's one of
several first-class attribute syntaxes we support, all of which are
more-or-less equally general.

As discussed on cfe-commits, we may want to revisit this if we start allowing
this syntax as an extension in C (or if C adopts the syntax), but hopefully
this diagnostic wording will be crystal clear to everyone in the mean time.