Turn ProjectionElem::Subtype into CastKind::Subtype

[beepster4096]

I noticed that drop elaboration can't, in general, handle ProjectionElem::SubType. It creates a disjoint move path that overlaps with other move paths. (Subslice does too, and I'm working on a different PR to make that special case less fragile.) If its skipped and treated as the same move path as its parent then MovePath.place has multiple possible projections. (It would probably make sense to remove all Subtype projections for the canonical place but it doesn't make sense to have this special case for a problem that doesn't actually occur in real MIR.)

The only reason this doesn't break is that Subtype is always the sole projection of the local its applied to. For the same reason, it works fine as a CastKind so I figured that makes more sense than documenting and validating this hidden invariant.

cc #112651, #133258

r? Icnr (bc you've been the main person dealing with Subtype it looks like)