IRGen: correct the typecast (NFC)

This seems unused currently, as attempting to query the DeclContext of
these types results in an immediate assertion that the kind of entity is
invalid.  These are non-root protocol conformances, use
`getProtocolConformance` instead of the copy-paste value of
`getRootProtocolConformance`.

Author: compnerd

lib/IRGen/Linking.cpp

@@ -988,8 +988,8 @@ DeclContext *LinkEntity::getDeclContextForEmission() const {
   case Kind::ReflectionAssociatedTypeDescriptor:
   case Kind::ProtocolWitnessTableLazyCacheVariable:
   case Kind::ProtocolWitnessTableLazyAccessFunction:
-    return getRootProtocolConformance()->getDeclContext();
-    
+    return getProtocolConformance()->getDeclContext();
+
   case Kind::TypeMetadata: {
     auto ty = getType();
     // Only fully concrete nominal type metadata gets emitted eagerly.