Refactor: have miri create an `Allocation` for the vtable

[crlf0710]

This is mentioned by @bjorn3 in #86291 (comment) .

The goal is this:

have miri create an Allocation for the vtable and then handle it like a normal constant in cg_{clif,llvm}?

Here's the mentoring notes:

The first step would be to refactor the vtable generation in miri to create an Allocation outside of the context of a Machine. In the place of the vtable codegen in cg_{clif,ssa} this function could be called followed by a call to whatever method is used to lower Allocations to backend constants. It may also be nice to add a map from trait + type -> allocation to tcx.alloc_map or something like that to replace the interning done by the backends.