Respect visibility & stability of inherent associated types
As discussed in #103621, this probably won't be the final location of the code that resolves inherent associated types. Still, I think it's valuable to push correctness fixes for this feature (in regards to visibility and stability).
Let me know if I should write a translatable diagnostic instead and if I should move the tests to `privacy/` and `stability-attribute/` respectively.
Fixes#104243.
````@rustbot```` label A-visibility F-inherent_associated_types
r? ````@cjgillot```` (since you reviewed #103621, feel free to reroll though)
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