blanket impl handling for not-yet-defined opaque types is not uwu

[lcnr]

cc rust-lang/rust#146329

see tests/ui/impl-trait/non-defining-uses/avoid-inference-constraints-from-blanket-2.rs

should revisit pre stabilization

ideally only use blanket impls if their only inference constraints are from using item bounds of opaques. Something something introduce MaybeCause::OpaqueTypeGuidance :3