Associated type redeclarations occasionally occur to push around
associated type witness inference. Suppress the warning about redeclarations
that add no requirements (i.e., have neither an inheritance nor a
where clause).
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.
When two associated types with the same name are on the same dependent
type T, introduce a same-type constraint between the the corresponding
potential archetypes. This eliminates ordering dependencies in the
archetype builder.
Fixes the reduced test case from rdar://problem/23149063, but doesn't
fully address the idea that we should be tracking associated type
redeclarations in a meaningful way.
