[Clang][Sema] Implement GCC -Wcast-function-type

Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable argument
list only the types of initial arguments that are provided are
considered. Any parameter of pointer-type matches any other
pointer-type. Any benign differences in integral types are ignored, like
int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The
function type void (*) (void) is special and matches everything, which
can be used to suppress this warning. In a cast involving pointer to
member types this warning warns whenever the type cast is changing the
pointer to member type. This warning is enabled by -Wextra.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D97831

GitOrigin-RevId: 217f0f735afec57a51fa6f9ab863d4713a2f85e2
6 files changed