avoid warning on explicit 'bool <- restricted' casts

avoid warning on explicit 'bool <- restricted' casts

Conversion to bool is special in C since this conversion
is essentially the result of the comparison with zero.
As such, some operations which are normally unsafe to
do with restricted types, like casting to an unrestricted
type, are in fact safe to do when converting to bool
and issuing a warning in those case is useless, confusing
and causes people to add useless casts in the code in
order to shut up the warning.

Fix this by catching explicit 'bool <- restricted type' casts
and not emit a warning like for others casts of restricted
type to non-restrictes ones.

Based-on-patch-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>