fix forward declared enums

[aalmkainzi]

mimics gcc/clang behavior:

enum E a; // error: incomplete enum

but this is ok:

enum E a;

enum E { A };

Also fixed cake not giving diagnosis with functions defs that contain incomplete types:

void foo(struct S) {}

the above code, cake used to error with no message, now gives:

error 740: function has incomplete parameter type
 1 |void foo(struct S) {}
   |     ~~~

however, this is allowed, as it should be:

enum E : int;
void foo(enum E);

[thradams]

I added the fixes manually