disallow 0-length bitfield for a union if --ccomp is passed

ccomp 2.0 doesn't support the code below:

$ cat foo.c
union {
  int : 0;
} a;
$ ccomp -fall -c union_zero_bitfield.c
union_zero_bitfield.c:1: Unsupported feature: bit field in struct or union

The failure message is a little bit mis-leading, because this code
can be compiled:

$ cat struct.c
struct {
  int : 0;
} a;
$ ccomp -fall -c struct.c ; echo $?
0