Add comment explaining why we skip

Author: ktoso

lib/IRGen/IRSymbolVisitor.cpp

@@ -104,7 +104,7 @@ class IRSymbolVisitorImpl : public SILSymbolVisitor {
   void addDispatchThunk(SILDeclRef declRef) override {
     auto entity = LinkEntity::forDispatchThunk(declRef);
 
-    // TODO: explain why
+    // distributed thunks don't need dispatch thunks.
     if (declRef.isDistributedThunk()) {
       auto afd = declRef.getAbstractFunctionDecl();
       if (afd && isa<ProtocolDecl>(afd->getDeclContext())) {