[Checkers] checking for SupportedAnnotationTypes

[Matt Papi]

Mahmood,

It looks like r2593 does exactly what (part of) r2584 does -- raising
an error about the presence of @SupportedAnnotationTypes -- though in
a different place. Was this intentional?

(Either way, I think SourceChecker is a better place for the check,
since that's where getSupportedAnnotationTypes() is overridden to
return "*".)

- Matt