Clang supports indirect call Control-Flow Integrity (CFI) sanitizers
(e.g. -fsanitize=cfi-icall), which enforce an exact type match
between a function pointer and the target function. Unfortunately,
Clang doesn't provide diagnostics that help developers avoid
function pointer assignments that can lead to runtime CFI
failures. -Wincompatible-function-pointer-types doesn't warn about
enum to integer mismatches if the types are otherwise compatible, for
example, which isn't sufficient with CFI.
Add -Wincompatible-function-pointer-types-strict, which checks for a
stricter function type compatibility in assignments and helps warn about
assignments that can potentially lead to CFI failures.