devel/libc++: Fix std::is_function<void() const> failed

In file included from /usr/include/c++/v1/string:439:
In file included from /usr/include/c++/v1/algorithm:624:
/usr/include/c++/v1/type_traits:433:76: error: no matching function for call to '__source'

: public integral_constant<bool, sizeof(__is_function_imp::__test<_Tp>(__is_function_imp::__source<_Tp>())) == 1>
                                                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/include/c++/v1/type_traits:438:14: note: in instantiation of template class 'std::1::libcpp_is_function<void (unsigned char *) const, false>' requested here

: public __libcpp_is_function<_Tp> {};
         ^

/usr/include/c++/v1/type_traits:443:97: note: in instantiation of template class 'std::1::is_function<void (unsigned char *) const>' requested here
template <class _Tp, class _Up> struct libcpp_is_member_function_pointer<_Tp _Up::*> : public is_function<_Tp> {};

                ^

/usr/include/c++/v1/type_traits:446:14: note: in instantiation of template class 'std::1::libcpp_is_member_function_pointer<void (flatbuffers::simple_allocator::*)(unsigned char *) const>' requested here

: public __libcpp_is_member_function_pointer<typename remove_cv<_Tp>::type> {};
         ^

PR: 215016
Submitted by: cpm
Reviewed by: dim (maintainer)
Approved by: dim (maintainer), feld (mentor, implicit)
Obtained from: https://reviews.llvm.org/D7573
MFH: 2016Q4