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

[Bill Wendling via cfe-commits]

bwendling wrote:

> Note the attribute is prevented on pointee types where the size isn't known at compile time. In particular pointee types that are:
> 
> * Incomplete (e.g. `void`) and sizeless types
> * Function types (e.g. the pointee of a function pointer)
> * Struct types with a flexible array member

I've been thinking about this restriction. Why is this necessary? My assumption was that applying `counted_by` to a pointer causes a bounds check on an index into the __pointer__ rather than its underlying type. So something like:

```c
struct foo;
struct bar {
  int a;
  int fam[] __counted_by(a);
};

struct x {
    void *p __counted_by(count);       // void * is treated like char *, I think.
    struct foo *f __counted_by(count); // sizeof(f) is the size of a general pointer.
    struct bar *b __counted_by(count); // a list of pointers to 'struct bar's should be okay.
    int count;
};
```

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