[clang-tidy] Resolve readability-else-after-return false positive for constexpr if.

Summary:
It fixes the false positive when using constexpr if and where else cannot be removed:

Example:
```
  if constexpr (sizeof(int) > 4)
    // ...
    return /* ... */;
  else // This else cannot be removed.
    // ...
    return /* ... */;
```

Reviewers: alexfh, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: lebedev.ri, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D53372

llvm-svn: 344785