[Clang][counted_by] Add support for 'counted_by' on struct pointers (llvm#137250)

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

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

If the pointer's type is 'void *', use the 'sized_by' attribute, which
works similarly to 'counted_by', but can handle the 'void' base type:

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

If the 'count' field member occurs after the pointer, use the
'-fexperimental-late-parse-attributes' flag during compilation.

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