Make withDict opaque to the specialiser

As pointed out in #21575, it is not sufficient to set withDict to inline after the typeclass specialiser, because we might inline withDict in one module and then import it in another, and we run into the same problem. This means we could still end up with incorrect runtime results because the typeclass specialiser would assume that distinct typeclass evidence terms at the same type are equal, when this is not necessarily the case when using withDict.

Instead, this patch introduces a new magicId, 'nospec', which is only inlined in CorePrep. We make use of it in the definition of withDict to ensure that the typeclass specialiser does not common up distinct typeclass evidence terms.