[CN] misleading error message concerning undefined struct arguments

[peterohanley]

#437 also concerns undefined structs but here cn permits them, then gives a misleading message about A!=A.

struct y {
    void* y;
};
void foo(struct s *a)
{
}
void bar(void)
{
    struct y a = {0};
    foo((struct s*)&a);
}

% cn verify incompatible.c
incompatible.c:14:9: error: constraint violation: incompatible pointer types passing 'struct s*' to parameter of type 'struct s*'
    foo((struct s*)&a);
        ^~~~~~~~~~~~~ 

[peterohanley]

If the structure is declared out of order with the struct this error will still happen and print the same message but now one s is the declared one and the other s is the undeclared one

% cat incompatible.c 
void foo(struct s *a)
{}
struct s {
    void* y;
};
void bar(void)
{
    struct s a = {0};
    foo((struct s*)&a);
}
% cn verify incompatible.c
incompatible.c:9:9: error: constraint violation: incompatible pointer types passing 'struct s*' to parameter of type 'struct s*'
    foo((struct s*)&a);
        ^~~~~~~~~~~~~