[Clang] [[gnu::weak, gnu::ifunc]] isn't diagnosed

Using both `[[gnu::weak]]` and `[[gnu::ifunc]]` is not supported. However, Clang silently drops the `[[gnu::weak]]` part instead of diagnosing the issue like GCC does.
For completeness, here is a simple reproducer:
```c++
extern "C" void* resolver() {
return nullptr;
}

[[gnu::weak, gnu::ifunc("resolver")]] void selector();
```

Start by compiling the provided C++ reproducer in Clang and tracing how the [[gnu::weak]] and [[gnu::ifunc]] attributes are handled. Done means Clang diagnoses their unsupported combination instead of silently dropping [[gnu::weak]], matching GCC's behavior.

Written by the indexing model from the issue text.