Preserve keyword comments

Summary: Print the concrete `extends` token for service inheritance instead of reconstructing it as plain text, so comments attached to that token are traversed and retained. Repeat for `performs`, `include`, and other tokens that are not captured in AST.

Reviewed By: hchokshi

Differential Revision: D110081316

fbshipit-source-id: e9c0a8bf57d61c6a599b5bc9f3cd5fad0d687ce9

Author: iahs