Having purpose attached to the contextual type element makes it much
easier to diagnose contextual mismatches without involving constraint
system state.
Nothing besides static member refs on protocols feature is currently
using transitive protocols, so instead of trying to infer them on every
step let's do that only for base type of a dot-syntax reference when
there are no other bindings for it.
Currently inference logic only checked direct equivalence class members
associated with a "work-in-progress" type variable, but each member can
have local equivalences as well that need to be accounted for.
Resolves: rdar://75978086
`PotentialBindings` lost most of its responsibilities,
and are no longer comparable. Their main purpose now
is binding and metadata tracking (introduction/retraction).
New `BindingSet` type is something that represents a set
of bindings at the current step of the solver.
