Rollup merge of #145620 - compiler-errors:fake-dyn-to-dyn, r=lcnr

Account for impossible bounds making seemingly unsatisfyable dyn-to-dyn casts

Fixes https://github.com/rust-lang/rust/issues/141806

When we have an impossible where clause like `dyn Trait<u8>: Sized`, this may make a dyn-to-dyn cast like `dyn Trait<()> -> dyn trait<u8>` to successfully type check as if it were a wide-to-thin ptr cast (discarding metadata):

https://github.com/rust-lang/rust/blob/16ad385579cebb6f7d53367c552661b6b51a4a02/compiler/rustc_hir_typeck/src/cast.rs#L862-L865

In borrowck, we are expecting that the only meaningful dyn-to-dyn cast to be a metadata-preserving wide-to-wide ptr cast, which requires that the principals of the dyn pointers are equal. Borrowck additionally assumes that these principals have already been proven equal *modulo regions*, and we thus ICE since `Trait<u8>` and `Trait<()>` do not unify:

https://github.com/rust-lang/rust/blob/16ad385579cebb6f7d53367c552661b6b51a4a02/compiler/rustc_borrowck/src/type_check/mod.rs#L1481-L1524

This PR fixes this ICE by checking whether the RHS of the cast is considered to be Sized in the environment of the MIR typeck, and if so then skipping over this dyn->dyn principal compatibility check.

r? `@lcnr` perhaps?

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