warn not error when SYCL_EXTERNAL applied to anonymous namespaces

LLNL Quicksilver (https://github.com/LLNL/Quicksilver) uses anonymous namespaces extensively for code that is only used within a single compilation unit, which is the modern C++ equivalent of static functions (https://stackoverflow.com/questions/154469/unnamed-anonymous-namespaces-vs-static-functions).

Given that anonymous namespaces only need to be resolved in a single compilation unit, I do not see why the compiler can't support these. We should support as a SYCL extension ASAP and propose to Khronos.

I just spent the last hour fixes ~20 instances of this warning.
```
error: 'sycl_device' attribute cannot be applied to a static function or function in an anonymous namespace
```
I don't think you need a reproducer, but https://github.com/jeffhammond/Quicksilver/tree/29f799ed09d16145aa63753d7e8527946a901f6d has a non-minimal one.