[clang] [Clang][Sema] Disallow applying `onwership_returns` to functions that return non-pointers (PR #99564)

[via cfe-commits]

================
@@ -1308,6 +1308,10 @@ Crash and bug fixes
 Improvements
 ^^^^^^^^^^^^
 
+- Improved handling of ``__attribute__((ownership_returns(class, idx)))``. Now clang
+  reports an error if attribute is attached to a function that returns non-pointer
+  value
----------------
Sirraide wrote:

```suggestion
- Improved the handling of the `ownership_returns` attribute. Now, Clang reports an 
  error if the attribute is attached to a function that returns a non-pointer value. 
  Fixes (#GH99501)
```

https://github.com/llvm/llvm-project/pull/99564