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

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

This looks good to me as far as the current use/behavior is concerned. That being said, I think these annotations make sense to some non-pointer types as well, e.g., integer handles to other system resources. I did not find any test/functionality for this scenario, so I am OK with this change for now, but this is something that we want to refine if new functionality is introduced in the future. 

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