[clang] [clang][Sema] Combine fallout warnings to just one warning (PR #127546)

[via cfe-commits]

Sirraide wrote:

> https://godbolt.org/z/4EjMGoqTn
> 
> So, That is what I'd like to expect :D

What we’re currently emitting is more or less:
```c++
namespace diag {
namespace FunModes {
enum {
    Function,
    // ...
};
}
}
```

https://github.com/llvm/llvm-project/pull/127546