Not support anonymous union in struct

[tang4530]

[ladisgin]

Hello @tang4530, thanks for request.

Description
UTBotCpp don't generate test for struct with anonymous union.

typedef struct {
  int a;
  union {
    int b;
    int c;
  } u;
} Info;

int inner(Info l) {
  if (l.a == 1) {
    l.u.b = 4;
    return 0;
  }
  if (l.a == 2) {
    l.u.c = 5;
    return 1;
  }
  return -1;
}

Expected behavior
Test will generated same as struct with named union.