Figure out a more idiomatic way to check if we should generate comments.

[emilio]

in #444 I pretty much wrapped everything in an if block, but @fitzgen suggested it may be a better idea to get the potential comment holders the context so they can check themselves.

[amanjeev]

Adding more context to this during triage:

Code review comment by @fitzgen : https://github.com/rust-lang/rust-bindgen/pull/444/files#r98064839

This is a whole lot of copy paste... What if we made Item::comment and Field::comment take the context and check themselves? The earlier we push this check, we could even potentially avoid keeping the comments around in memory.

@kulp @emilio can any of you please add a tag about "good first issue". That is missing in the list. Or let me know if any other tag is used in place of that.

[kulp]

@kulp @emilio can any of you please add a tag about "good first issue".

@amanjeev sorry to be unavailable for such long periods; but I think you / @pvdrz have all the rights you need now. Thanks for handling this.