Reimplement isSelfDerivedSource() in terms of the new
visitPotentialArchetypesAlongPath(). Aside from being clearer and
shorter, this formulation eliminates some hackery (the direct
"NestedType" lookup) that was mostly masking a bug in
updateNestedTypeForConformance() where we created a
nested-type-name-match source with an incorrect source for the
purposes of concretizing the nested type.
Fixes SR-4458 / rdar://problem/31375569.