Move __vector long deprecation checking into DeclSpec::Finish

__vector long is deprecated, but __vector long long is not. As a result, we
cannot check for __vector long (to issue the deprecation warning) as we parse
the type because we need to know how many 'long's we have first.
DeclSpec::Finish seems like a more-appropriate place to perform the check
(which places it with several other similar Altivec vector checks).

Fixes PR20720.