SIL: Fix SILFunctionType::getDefaultWitnessMethodProtocol() for class-constrained 'self' archetypes

Right now witness_method functions either have a concrete type as the
'self' argument or a type parameter with a single protocol conformance
constraint.

Soon we will also have class-constrained archetypes here, so tweak the
assertion here to detect this case.

Author: slavapestov

lib/SIL/SILFunctionType.cpp

@@ -100,6 +100,9 @@ SILFunctionType::getDefaultWitnessMethodProtocol(ModuleDecl &M) const {
   assert(getRepresentation() == SILFunctionTypeRepresentation::WitnessMethod);
   auto selfTy = getSelfInstanceType();
   if (auto paramTy = dyn_cast<GenericTypeParamType>(selfTy)) {
+    auto superclass = GenericSig->getSuperclassBound(paramTy, M);
+    if (superclass)
+      return nullptr;
     assert(paramTy->getDepth() == 0 && paramTy->getIndex() == 0);
     auto protos = GenericSig->getConformsTo(paramTy, M);
     assert(protos.size() == 1);