SILGen: Unwrap inherited conformance when emitting witness thunk

I'm about to fix RequirementEnvironment to correctly use
inherited conformances.

Author: slavapestov

lib/SILGen/SILGenType.cpp

@@ -797,6 +797,15 @@ SILFunction *SILGenModule::emitProtocolWitness(
     }
   }
 
+  ProtocolConformance *manglingConformance = nullptr;
+  if (conformance.isConcrete()) {
+    manglingConformance = conformance.getConcrete();
+    if (auto *inherited = dyn_cast<InheritedProtocolConformance>(manglingConformance)) {
+      manglingConformance = inherited->getInheritedConformance();
+      conformance = ProtocolConformanceRef(manglingConformance);
+    }
+  }
+
   // Lower the witness thunk type with the requirement's abstraction level.
   auto witnessSILFnType = getNativeSILFunctionType(
       M.Types, TypeExpansionContext::minimal(), AbstractionPattern(reqtOrigTy),
@@ -805,8 +814,6 @@ SILFunction *SILGenModule::emitProtocolWitness(
 
   // Mangle the name of the witness thunk.
   Mangle::ASTMangler NewMangler;
-  auto manglingConformance =
-      conformance.isConcrete() ? conformance.getConcrete() : nullptr;
   std::string nameBuffer =
       NewMangler.mangleWitnessThunk(manglingConformance, requirement.getDecl());
   // TODO(TF-685): Proper mangling for derivative witness thunks.