IRGen: Instantiate existential metadata through the runtime.

When we need a reference to protocol or protocol composition type metadata, ask for it through the runtime, instead of referencing statically-emitted protocol metadata.

Swift SVN r9871

lib/IRGen/IRGenModule.cpp

@@ -113,6 +113,8 @@ IRGenModule::IRGenModule(ASTContext &Context,
     Int32Ty                 // flags
   });
   
+  ProtocolDescriptorPtrTy = ProtocolDescriptorStructTy->getPointerTo();
+  
   // A tuple type metadata record has a couple extra fields.
   auto tupleElementTy = createStructType(*this, "swift.tuple_element_type", {
     TypeMetadataPtrTy,      // Metadata *Type;