Changes to satisfy analyzer_use_new_elements lint.

[davidmorgan]

This change is mostly by @stereotype441 with a few tweaks on top by me :) ... it passes tests in google3.

This makes source_gen comply with the lint, except for one line where I add an ignore to continue calling generateForAnnotatedElement.

After this I think the next steps are:

1. migrate all users in google3 to source_gen element2 API
2. next source_gen PR: copy element2 methods removing the "2" to give the final API we want, but leaving the APIs with "2" in the name in place
3. migrate all users in google3 to source_gen renamed element2->element API
4. next+1 source_gen PR: remove all methods with "2" in the name

how does that sound? Thanks :)