[Clang][counted_by] Add support for 'counted_by' on struct pointers

The 'counted_by' attribute is now available for structs. It generates
code for sanity checks as well as __builtin_dynamic_object_size()
calculations. For example:

  struct annotated_ptr {
    int count;
    void *buf __attribute__((counted_by(count)));
  };

The attribute can be applied to a 'void *'. In that case, use the
'sized_by' attribute instead:

  struct annotated_ptr {
    void *buf __attribute__((sized_by(count)));
    size_t count;
  };

The 'count' field member may occur after the pointer. If it does, use
the '-fexperimental-late-parse-attributes' flag (which will hopefully be
made the default in future Clang versions).

Note that 'counted_by' cannot be applied to a pointer to an incomplete
type, because the size isn't known.

  struct foo;
  struct annotated_ptr {
    int count;
    struct foo *buf __attribute__((counted_by(count))); /* invalid */
  };

Signed-off-by: Bill Wendling <[email protected]>

Author: bwendling