[Bug preprocessor/103027] Implement warning for homoglyphs in identifiers [CVE-2021-42694]

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103027

--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Initial version of patch posted for discussion to:
  https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583039.html