__builtin_*_overflow crashes with __intcap types

We should either emit an error or perform the computation on the address part.

For example https://cheri-compiler-explorer.cl.cam.ac.uk/z/fj58sb:
```c
#include <stdlib.h>

__intcap test(__intcap a, __intcap b) {
   __intcap ret;
   if (__builtin_add_overflow(a, b, &ret))
       abort();

   return ret;
}
```