Add chalk rules related to associated type defs

[scalexm]

Fixes#50580.

As GATs are currently not implemented in rustc, we don't need the rule Implied-Bound-From-AssocTy for the moment. Indeed, this is basically:

traitFoo{typeAssoc:Bound;}// forall<Self> { FromEnv(<Self as Foo>::Assoc: Bound) :- FromEnv(Self: Foo) }

but in the absence of GATs, the previous bound can be expressed as:

traitFoowhere <SelfasFoo>::Assoc:Bound{}

hence the above rule will come automatically with implied bounds from where clauses.

Similarly, the rule Implied-WC-From-AssocTy is not needed right now because associated types do not currently support where clauses.

r? @nikomatsakis

[nikomatsakis]

r=me once rebased — actually @scalexm if you want, we could land this together with #54866 to save bors time (i.e., close #54866 and just land this)

[scalexm]

@bors r=nikomatsakis

[bors]

📌 Commit 282559c has been approved by nikomatsakis