[libc++] Add missing includes to optional

After commit 0a4aa8a1 we see failures
like:
optional:722:7: error: missing '#include <__type_traits/conjunction.h>';
'_And' must be declared before it is used
and:
optional:683:46: error: missing '#include
<__type_traits/disjunction.h>'; '_Or' must be declared before it is used
      using __check_constructible_from_opt = _Or<

Adding these here fixes that.
I'm not familiar with the libcxx codebase, will ask the author to take a
look too.