Introduce a warning about redeclaring the associated types from an
inherited protocol in the protocol being checked:
* If the new declaration is an associated type, note that the
declaration could be replaced by requirements in the protocol's
where clause.
* If the new declaration is a typealias, note that it could be
replaced by a same-type constraint in the protocol's where clause.
Move the storage for the protocols to which a particular potential
archetype conforms into EquivalenceClass, so that it is more easily
shared. More importantly, keep track of *all* of the constraint
sources that produced a particular conformance requirement, so we can
revisit them later, which provides a number of improvements:
* We can drop self-derived requirements at the end, once we've
established all of the equivalence classes
* We diagnose redundant conformance requirements, e.g., "T: Sequence"
is redundant if "T: Collection" is already specified.
* We can choose the best path when forming the conformance access
path.
This is a generic signature that stores exactly the requirements that a
protocol decl introduces, not letting them be implied by the Self :
Protocol requirement, nor storing any requirements introduced by the
protocols requirements.
Specifically, suppose we have
protocol Foo {}
protocol Bar {}
protocol Baz {
associatedtype X : Foo
}
protocol Qux: Baz {
associatedtype X : Bar
}
The normal generic signature and (canonical) protocol requirement
signature of `Baz` will be, respectively
<Self where Self : Baz>
<Self where Self : Baz, Self.X : Foo>
And for `Qux`, they will be:
<Self where Self : Qux>
<Self where Self : Qux, Self : Baz, Self.X : Bar>
Note that the `Self.X : Foo` requirement is not listed.
For the moment, this is unused except for `-debug-generic-signatures`.
