[Bug preprocessor/103026] Implement warning for Unicode bidi override characters [CVE-2021-42574]

[dmalcolm at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103026

--- Comment #8 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
For refererence, here's a demo showing the colorized output on Compiler
Explorer:
  https://godbolt.org/z/1sohErzWz