Fix duplicate unbound name errors

Summary: When naming a hint, we'd call `Env.class_name` on it twice
here, causing it to get looked up and canonicalized more than once --
and thus insert an error more than once if it was wrong, such as the
case in the github issue linked below. This is hard to test for --
`hh_server --check` will unique-ify errors, and `hh_single` will only
take the first one. Fixing the latter is beyond what I want to deal with
right now, so this manually tested fix it is.

Fixes https://github.com/facebook/hhvm/issues/3294

Reviewed By: @int3, @elgenie

Differential Revision: D1677992

Signature: t1:1677992:1415857065:675182d27592e614b5ee8864648b3367a2ef01e3