The identifier in a reference is used to check for a doc comment.
Implicits do not have an ident, so do not use that to look for a doc
comment.
Also set the context.Context for the renamer.
Change-Id: I085d9e6c11d919222592dcb6fb30982eeb0fc7cd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184042
Reviewed-by: Rebecca Stambler <rstambler@golang.org>