Properly handle `Pin<&mut dyn* Trait>` receiver in codegen
This ensures we can actually await a `dyn* Future`, which seems important for async fn in dyn trait.
Also, disable `dyn*` trait upcasting. It's not exactly complete right now, and can cause strange ICEs for no reason -- nobody's using it either. I thought it was cute to implement when I did it, but I didn't think about how it interacts structurally with `CoerceUnsized` correctly.
Fixes#104794, presumably removing `dyn*` upcasting and its `CoerceUnsized` issues does the trick.
b43c2e7cd9