Chrono: comment version 2 setting when building v1_ tests.

svn path=/trunk/libs/chrono/; revision=83841
Chrono: use libc++ constexpr if _LIBCPP_CONSTEXPR is defined #8367.

svn path=/trunk/boost/chrono/; revision=83842
Chrono: cleanup.

svn path=/trunk/boost/chrono/; revision=83843