[clang-tidy] Fix false positive in `readability-else-after-return` on `return` jumped over by `goto` (#186370)

Given this code:

```cpp
if (...) {
  goto skip_over_return;
  return;
skip_over_return:
  foo();
} else {
  ...
}
```

...the check suggests removing the `else`, which is not a valid
transformation. This is because it looks at *all* the substatements of
the then-branch for interrupting statements. This PR changes it to only
look at the *final* substatement.

Technically, this introduces a false negative on code like this:

```cpp
if (...) {
  return;
  dead_code();
} else { // <-- Could in theory remove this 'else'
  ...
}
```

But, that code is objectively bad, so I don't think we're losing
anything.

This change has the side effect of making the check a bit more general;
it now recognizes attributed interrupting statements (e.g.
`[[clang::musttail]] return f();`).