Unify intrinsics body handling in StableMIR
rust-lang/rust#120675 introduced a new mechanism to declare intrinsics which will potentially replace the rust-intrinsic ABI.
The new mechanism introduces a placeholder body and mark the intrinsic with `#[rustc_intrinsic_must_be_overridden]`.
In practice, this means that a backend should not generate code for the placeholder, and shim the intrinsic.
The new annotation is an internal compiler implementation, and it doesn't need to be exposed to StableMIR users.
In this PR, we unify the interface for intrinsics marked with `rustc_intrinsic_must_be_overridden` and intrinsics that do not have a body.
Fixes https://github.com/rust-lang/project-stable-mir/issues/79
r? ``@oli-obk``
cc: ``@momvart``
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