Invalid bounds declaration allowed in checked scope

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#639

---

The current version of clang allows the following declarations:

#include<stdio_checked.h>
#pragma CHECKED_SCOPE ON
#pragma BOUNDS_CHECKED ON
struct foo{
   unsigned l;
} glo;
int modify_bounds_variable(_Array_ptr<char> p:count(2)) {
  char a;
  // here we go out of bounds.
  // here, we are changing the variable i.e., p used in a bounds declaration
  // THIS SHOULD NOT BE ALLOWED.
  p+=32;
  // this is out-of-bounds read
  a = p[0];
  putc(a, stdout);
  return 0;
}
int main(void) {
   _Array_ptr<char> p:count(12)= "heOOOOOOOOOO";
   _Ptr<struct foo> obj = &glo;
   // we are using a pointer dereference in a bounds declaration.
   // THIS SHOULD NOT BE ALLOWED.
   _Array_ptr<char> p2:count(obj->l) = NULL;
   modify_bounds_variable(p);
   return 0;
}

As one can see that the changes to p i.e., p += 32 should not be allowed. Also, the bounds declaration of containing pointer dereference i.e., _Array_ptr<char> p2:count(obj->l) should not be allowed as it requires reasoning about pointers.