[Bug middle-end/64463] Add warning: returns_nonnull attribute on a function compared against NULL

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64463

--- Comment #2 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
This is handled before reaching the middle-end, thus it would need to be
detected in the FE.