[clang-tools-extra] [clang-tidy] Fix readability-else-after-return for if statements appear in unbraced switch case labels (PR #181878)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

While we are here, how about handling labels (used for `goto`) as well?

---

We should handle fallthrough, which is common. You might write a new matcher to detect it.

```cpp
case 0:
case 1:
// arbitrary number of `case`
```

https://github.com/llvm/llvm-project/pull/181878