[llvm] [IR] Allow nofree on return values (PR #203491)

[Ralf Jung via llvm-commits]

================
@@ -1568,7 +1568,10 @@ Currently, only the following parameter attributes are defined:
     Notably, it is still possible to free the underlying object through a
     pointer that is not based on the argument.
 
-    This is not a valid attribute for return values.
+    For return values, this indicates that the underlying object cannot be freed
+    after this point (matching the semantics of the ``!nofree`` metadata). This
----------------
RalfJung wrote:

```suggestion
    after this point for the entire rest of the execution (matching the semantics of the ``!nofree`` metadata). This
```
To avoid any doubt

https://github.com/llvm/llvm-project/pull/203491