Print error if defined type is incomplete at end of translation unit

[ShivamSarodia]

For example, in the following code

int a[];
int main() { }

the array a should be implicitly allocated to have exactly one element, but GCC prints a warning in this case. Similarly, GCC rejects the following code:

struct S s;
int main() {}

and ShivyC should perhaps do the same rather than accepting it silently.