Print warning when declaring annotations with SOURCE retention via ITD

[kriegaex]

This is a follow-up issue for PR #196 and Bugzilla bug 366085.

The imperfect workaround for that bug simply ignores declared source-level annotations occurring in the AST. The better way to solve this problem would be to eliminate them from the AST when parsing the aspect at some point after the corresponding annotation type has been resolved and its retention level can be determined with certainty. In each case, a comprehensive weaver or compiler warning should be issued and the AST cleaned up (or not polluted in the first place, whichever method is applicable).

@aclement, like I said, I would need your guidance in order to fix this problem properly, which is why for now I shall leave it at merging the workaround.