[Bug c++/55077] implement and enable by default -Wliteral-conversion

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
Created attachment 33637
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33637&action=edit
untested patch

Untested patch. Bonus points if we show the value before and after conversion
like clang does.