Remove unnecessary layout assertions for object-safe receivers
The soundness of `DispatchFromDyn` relies on the fact that, like all other built-in marker-like layout traits (e.g. `Sized`, `CoerceUnsized`), the guarantees that they enforce in *generic* code via traits will result in assumptions that we can rely on in codegen.
Specifically, `DispatchFromDyn` ensures that we end up with a receiver that is a valid pointer type, and its implementation validity recursively ensures that the ABI of that pointer type upholds the `Scalar` or `ScalarPair` representation for sized and unsized pointees, respectively.
The check that this layout guarantee holds for arbitrary, possibly generic receiver types that also may exist in possibly impossible-to-instantiate where clauses is overkill IMO, and leads to several ICEs due to the fact that computing layouts before monomorphization is going to be fallible at best.
This PR removes the check altogether, since it just exists as a sanity check from very long ago, 6f2a161b1b.
Fixes#125810Fixes#90110
This PR is an alternative to #136195. cc `@adetaylor.` I didn't realize in that PR that the layout checks that were being modified were simply *sanity checks*, rather than being actually necessary for soundness.
5da750001d