prevent opaque types from appearing in impl headers
cc `@lqd`
opaque types are not distinguishable from their hidden type at the codegen stage. So we could either end up with cases where the hidden type doesn't implement the trait (which will thus ICE) or where the hidden type does implement the trait (so we'd be using its impl instead of the one written for the opaque type). This can even lead to unsound behaviour without unsafe code.
Fixes https://github.com/rust-lang/rust/issues/86411.
Fixes https://github.com/rust-lang/rust/issues/84660.
rebase of #87382 plus some diagnostic tweaks
Dylan DPC
e96304b73d