type_traits (__add_lvalue_reference_helper, [...]): As an optimization, rewrite condition (avoid is_function).

2009-03-14  Paolo Carlini  <paolo.carlini@oracle.com>

	* include/std/type_traits (__add_lvalue_reference_helper,
	__add_rvalue_reference_helper): As an optimization, rewrite
	condition (avoid is_function).

From-SVN: r144868