Fix extra commentability on prototype return value

lib/CliftEmitC/CEmitter.cpp

@@ -82,10 +82,18 @@ private:
     return Name;
   }
 
-  std::optional<ptml::CTokenEmitter::Region> commentableReturnValueGuard() {
+  std::optional<ptml::CTokenEmitter::Region>
+  commentableReturnValueGuard(DeclaratorInfo const *Declarator) {
     if (OutermostFunctionType == nullptr)
       return std::nullopt;
 
+    // Only emitting the return value guard when parameters are available might
+    // look weird at the first sight, but when we are emitting a prototype
+    // without parameter information (the typedef), it's weird to allow adding
+    // comments to a return type that will show up elsewhere.
+    if (not Declarator->Parameters)
+      return std::nullopt;
+
     auto FTHandle = OutermostFunctionType.getHandle();
     auto FTLoc = pipeline::locationFromString(revng::ranks::TypeDefinition,
                                               FTHandle);
@@ -141,7 +149,7 @@ private:
     }
 
     {
-      auto Guard = commentableReturnValueGuard();
+      auto Guard = commentableReturnValueGuard(Declarator);
 
       // Recurse through the declaration, pushing each level onto the stack
       // until a terminal type is encountered. Primitive types as well as