add dependent_false

always false idiome
dependent_false.hpp
```
// Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
// Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_DEPENDENT_FALSE_HPP
#define BOOST_TYPE_TRAITS_DEPENDENT_FALSE_HPP

#include

namespace boost
{
template struct dependent_false : public false_type{};

#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template inline constexpr bool dependent_false_v = dependent_false::value;
#endif
} // boost

#endif // BOOST_TYPE_TRAITS_DEPENDENT_FALSE_HPP
```

Review the proposed dependent_false.hpp snippet and the surrounding Boost.TypeTraits header layout. Done means the new header provides dependent_false and its conditional dependent_false_v as described, with the shown license and include guard preserved.

Written by the indexing model from the issue text.