Rollup merge of #139421 - compiler-errors:upcast-no-principal-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of #114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe82563146325674b8d437f9f9f6e703703, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in #126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes #139418

r? ````@oli-obk```` (or anyone)

Author: Zalathar