Conformances that discoverably for a type parameter via a superclass
requirement occupy a bit of a gray area in the type checker and
generic signature handling right now: we have a type parameter (so the
paths that handle concrete conformances don't kick in), but the
conformance itself isn't modeled as a requirement because it is
available via lookup. Teach SubstitutionMap's conformance lookup to
detect this case and perform conformance lookup in the generic
signature (i.e., falling back to global lookup) in this case,
replicating the hack that IRGen uses to address the same issue when
accessing the conformance (see GenArchetype.cpp's TODO where we lookup
a conformance on the superclass). The removal of that hack (as well as
this one) are tracked by rdar://problem/34609744.
For now, fixes SR-7072 / rdar://problem/37904576.
