This changes attr_form_is_block to be a method. This is done
separately because, unlike the other attribute functions,
attr_form_is_block had special handling for the case where the
argument was NULL. This required auditing each call site; in most
cases, NULL was already ruled out, but in a few spots, an additional
check was needed.
gdb/ChangeLog
2020-02-08 Tom Tromey <tom@tromey.com>