Auto merge of #57352 - arielb1:no-manual-markers, r=nikomatsakis
forbid manually impl'ing one of an object type's marker traits
This shouldn't break compatibility for crates that do not use
`feature(optin_builtin_traits)`, because as the test shows, it is
only possible to impl a marker trait for a trait object in the crate the
marker trait is defined in, which must define
`feature(optin_builtin_traits)`.
Fixes #56934.
r? @nikomatsakis