translate-c: casting away const fails

[tadeokondrak]

static void function2(int *a) {
	const int *b = a;
}

static void function(const int *a) {
	function2((int *)a);
}

const a: c_int = 0;
function(&a);

/home/tdeo/.cache/zig/stage1/o/.../cimport.zig:7:15: error: cast discards const qualifier
    function2(@ptrCast([*c]c_int, @alignCast(@alignOf(c_int), a)));

Motivation: Wayland add_listener functions cast their struct of function pointer arguments to void (**)(void) before passing them to another function, which then stores it in a const field and never modifies through it.

[andrewrk]

Related: it will be nice to have test coverage for these kinds of bugs with #4018.