rustdoc: synthetic auto: filter out clauses from the implementor's ParamEnv
... not just the elaborated clauses.
Fixes another regression introduced by me in #123340, oops!
Fixes https://github.com/rust-lang/rust/pull/123340#issuecomment-2034195786, cc ``@tamird.``
An earlier local iteration of branch `rustdoc-simplify-auto-trait-impl-synth` (PR #123340) contained a fix for issue #111101 before I decided to limit the scope. I must've introduced this bug when manually reverting that part of the code.
r? ``@GuillaumeGomez`` or rustdoc