3499: Resolve `Self::AssocTy` in impls r=matklad a=flodiebold
To do this we need to carry around the original resolution a bit, because `Self`
gets resolved to the actual type immediately, but you're not allowed to write
the equivalent type in a projection. (I tried just comparing the projection base
type with the impl self type, but that seemed too dirty.) This is basically how
rustc does it as well.
Fixes#3249.
Co-authored-by: Florian Diebold <florian.diebold@freiheit.com>
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