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

[Kees Cook via cfe-commits]

kees wrote:

> Consider this example. It tries to illustrate why putting `__counted_by()` on a pointer to a structs containing flexible array members doesn't make sense.
> 
> ```c
> struct HasFAM {
>     int count;
>     char buffer[] __counted_by(count); // This is OK
> };
> 
> struct BufferOfFAMS {
>     int count;
>     struct HasFAM* fams __counted_by(count); // This is invalid
> };
> ```

Agreed: structs with flexible array members must be considered to be singletons. This property is actually important for  being able to have `__builtin_dynamic_object_size()` work on pointers to flexible array structs. i.e.:

```
size_t func(struct foo *p)
{
    return__builtin_dynamic_object_size(p, 0);
}
```

This must always return `SIZE_MAX` for fixed-size arrays since the pointer may be in the middle of a larger array of `struct foo`s, but if it is a struct with a flexible array marked with `counted_by`, then we know deterministically what the size is, since it must be a single complete object.

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