Refactor vtable codegen
This refactor the codegen of vtables of miri interpreter, llvm, cranelift codegen backends.
This is preparation for the implementation of trait upcasting feature. cc #65991
Note that aside from code reorganization, there's an internal behavior change here that now InstanceDef::Virtual's index now include the three metadata slots, and now the first method is with index 3.
cc `@RalfJung` `@bjorn3`
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