fix def collector for impl trait
fixes#77329
We now consistently make `impl Trait` a hir owner, requiring some special casing for synthetic generic params.
r? `@eddyb`
This allows us to avoid synthesizing tokens in `prepend_attr`, since we
have the original tokens available.
We still need to synthesize tokens when expanding `cfg_attr`,
but this is an unavoidable consequence of the syntax of `cfg_attr` -
the user does not supply the `#` and `[]` tokens that a `cfg_attr`
expands to.
