bug#31676: 27.0.50; More helpful error message for unescaped character literals

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sat, 9 Jun 2018 19:12:30 +0200
> Cc: larsi@gnus.org, 31676@debbugs.gnu.org
> 
>  Sounds okay, but can you tell why you implemented
>  lread--unescaped-character-literals in C?  If that's because you need
>  to call it from load_warn_unescaped_character_literals, then C
>  functions can call Lisp functions with no problems, we have several
>  examples of that in the sources.  AFAICT, the C implementation is just
>  a "transliteration" of straightforward Lisp code, so it reads strange.
> 
> The function uses an uninterned variable, so it has to be in C. I think 
> that's slightly better than interning the
> variable and having some Lisp function access it (the latter would have one 
> additional internal symbol). 

Why does it need an uninterned variable?  And if it does, why cannot
it create a symbol that is not in obarray?