Replace #[rustc_do_not_implement_via_object] with #[rustc_dyn_incompatible_trait], which makes the marked trait dyn-incompatible.

Removes the attribute from `MetaSized` and `PointeeSized`, with a special case in the trait solvers for `MetaSized`.

`dyn MetaSized` is a perfectly cromulent type, and seems to only have had #[rustc_do_not_implement_via_object] so the builtin object
candidate does not overlap with the builtin MetaSized impl that all `dyn` types get.
Resolves this with a special case by checking `is_sizedness_trait` where the trait solvers previously checked `implement_via_object`.

`dyn PointeeSized` alone is rejected for other reasons (since `dyn PointeeSized` is considered to have no principal trait because `PointeeSized`
is removed at an earlier stage of the compiler), but `(dyn PointeeSized + Send)` is valid and equivalent to `dyn Send`.

Add suggestions from code review

Update compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs and tests

Co-authored-by: lcnr <rust@lcnr.de>