[build] Add IWYU mapping for boost's type_traits

boost/move/detail/type_traits.hpp is a private Boost.Move
implementation header providing pre-C++11 polyfills for type
predicates (is_trivially_destructible, is_nothrow_move_constructible,
etc.). IWYU's template instantiation tracing can surface it when
std algorithms move types that have opted into Boost.Move emulation,
even though no user code directly uses anything from it.

Add a mapping in boost-extra.imp that redirects it to the standard
<type_traits> header, which is its C++11 public equivalent and is
already included in the affected translation units. This prevents
IWYU from incorrectly suggesting a direct include of a Boost
internal detail header.

Change-Id: I4fa5d5d89f9aea049d64e111f77ac10a792d0463
Reviewed-on: http://gerrit.cloudera.org:8080/24617
Reviewed-by: Gabriella Lotz <lotzgabriella@gmail.com>
Reviewed-by: Zoltan Chovan <zchovan@cloudera.com>
Tested-by: Zoltan Chovan <zchovan@cloudera.com>