Reported by @loic-sharma.
The tl;dr is it should be OK to have `\r\n` (CRLF) endings if the header
guard is otherwise correct.
This minor refactor (and test) discounts the existence of `\r` when
determining the name of a header guard, i.e.:
```h
#ifndef FLUTTER_MATAN_WHY_H_
```
... is now (correctly) considered a value of `FLUTTER_MATAN_WHY_H_` not
`FLUTTER_MATAN_WHY_H_\r`.
bf073d4352