libstdc++: Fix laziness of __and/or/not_

libstdc++: Fix laziness of __and/or/not_

r13-2230-g390f94eee1ae69 redefined the internal logical operator traits
__and_, __or_ and __not_ as alias templates that directly resolve to
true_type or false_type.  But it turns out using an alias template here
causes the traits to be less lazy than before because we now compute the
logical result immediately upon _specialization_ of the trait, and not
later upon _completion_ of the specialization.

So for example, in

  using type = __and_<A, __not_<B>>;

we now compute the conjunction and thus instantiate A even though we're
in a context that doesn't require completion of the __and_.  What's
worse is that we also compute the inner negation and thus instantiate B
(for the same reason), independent of the __and_ and the value of A!
Thus the traits are now less lazy and composable than before.

Fortunately, the fix is cheap and straightforward: redefine these traits
as class templates instead of as alias templates so that computation of
the logical result is triggered by completion, not by specialization.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__or_, __and_, __not_): Redefine as a
	class template instead of as an alias template.
	* testsuite/20_util/logical_traits/requirements/short_circuit.cc:
	Add more tests for conjunction and disjunction.  Add corresponding
	tests for __and_ and __or_.