Auto merge of #115025 - ouz-a:ouz_testing, r=lcnr

Make subtyping explicit in MIR

This adds new mir-opt that pushes new `ProjectionElem` called `ProjectionElem::Subtype(T)` to `Rvalue` of a subtyped assignment so we can unsoundness issues like rust-lang/rust#107205

Addresses rust-lang/rust#112651

r? `@lcnr`