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
I also added `// skip-codegen` to each one, to address potential concerns
that this change would otherwise slow down our test suite spending time
generating code for files that are really just meant to be checks of
compiler diagnostics.
(However, I will say: My preference is to not use `// skip-codegen` if
one can avoid it. We can use all the testing of how we drive LLVM that
we can get...)
(Updated post rebase.)
