analyzer kernel plugin: implement __check_object_size [PR112927]

PR analyzer/112927 reports a false positive from -Wanalyzer-tainted-size
seen on the Linux kernel's drivers/char/ipmi/ipmi_devintf.c with the
analyzer kernel plugin.

The issue is that in:

(A):
  if (msg->data_len > 272) {
    return -90;
  }

(B):
  n = msg->data_len;
  __check_object_size(to, n);
  n = copy_from_user(to, from, n);

the analyzer is treating __check_object_size as having arbitrary side
effects, and, in particular could modify msg->data_len.  Hence the
sanitization that occurs at (A) above is treated as being for a
different value than the size obtained at (B), hence the bogus warning
at the call to copy_from_user.

Fixed by extending the analyzer kernel plugin to "teach" it that
__check_object_size has no side effects.

gcc/testsuite/ChangeLog:
	PR analyzer/112927
	* gcc.dg/plugin/analyzer_kernel_plugin.c
	(class known_function___check_object_size): New.
	(kernel_analyzer_init_cb): Register it.
	* gcc.dg/plugin/plugin.exp: Add taint-pr112927.c.
	* gcc.dg/plugin/taint-pr112927.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>