[clang] [BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C (PR #90786)

[Bill Wendling via cfe-commits]

================
@@ -6534,6 +6536,15 @@ def err_counted_by_attr_refer_to_union : Error<
   "'counted_by' argument cannot refer to a union member">;
 def note_flexible_array_counted_by_attr_field : Note<
   "field %0 declared here">;
+def err_counted_by_attr_pointee_unknown_size : Error<
+  "'counted_by' cannot be applied a pointer with pointee with unknown size "
+  "because %0 is %select{"
+    "an incomplete type|"  // CountedByInvalidPointeeTypeKind::INCOMPLETE
+    "a sizeless type|"     // CountedByInvalidPointeeTypeKind::SIZELESS
+    "a function type|"     // CountedByInvalidPointeeTypeKind::FUNCTION
----------------
bwendling wrote:

I'm a bit confused why a function type is excluded. Isn't it just a pointer? @kees can correct me, but I think the point of `counted_by` on a pointer is that it could be a list of pointers, and we don't want to allow someone to access beyond that list:

```c
struct s {
  int count;
  int *ptr __counted_by(count);
};

struct s *alloc(size_t num_elems) {
  struct s *p = malloc(sizeof(struct s));

  p->count = num_elems;
  p->ptr = calloc(sizeof(int), num_elems);
  return p;
}
```

If that's the case, then any pointer should be okay.

https://github.com/llvm/llvm-project/pull/90786