Diagnose [[clang::lifetimebound]] on functions whose return value cannot refer to any object

`[[clang::lifetimebound]]` on a parameter is meaningful only when the function's return value can refer to another object. When the return type is something like `int` or an enum, the returned value cannot carry a borrow, and the attribute has no effect:

```cpp
int getInt(const std::string &s [[clang::lifetimebound]]);
```

We should emit a diagnostic for this case.

cc @usx95