When we determine that a potential archetype's stated conformance to a
protocol is redundant because the superclass of that potential
archetype already conforms to that conformance, mark this conformance
as "inherited". Inherited conformances are dropped when building both
the archetype (because the conformance is recoverable via the
superclass) and when building the generic signature.
Previously, the latter occurred (because these were classified as
"redundant"), but the former did not, leading to inconsistencies in
the archetypes built from an explicitly-written "where" clause
vs. ones reconstituted from the generic signature.
Pair-programmed with Arnold. Fixes rdar://problem/29223093.
