[IR] Do not set `none` for function uwtable (#93387)

This avoids the pitfall where we set the uwtable to none:
```
func.setUWTableKind(llvm::UWTableKind::None)
```
`Attribute::getAsString()` would see an unknown attribute and fail an
assertion. In this patch, we assert that we do not see a None uwtable
kind.

This also skips the check of `UWTableKind::Async`. It is dominated by
the check of `UWTableKind::Default`, which has the same enum value
(nfc).