[clangd] auto including internal header when using `unreachable`

When auto completing `unreachable`, clangd auto includes `__stddef_unreachable.h`, even if `stddef.h` is already included.

```C
#include <__stddef_unreachable.h>
#include

void foo()
{
unreachable();
}
```

Start in clangd's auto-completion and include-insertion behavior, reproducing the example with __stddef_unreachable.h followed by stddef.h. Trace how unreachable is resolved when stddef.h is already present. Done means completion no longer adds the internal header in this case, while the existing unreachable completion still works.

Written by the indexing model from the issue text.