[libc++] Teach is_integral, is_[un]signed and make_[un]signed about __[u]int128_t

This commit also adds tests for std::numeric_limits<__[u]int128_t>.

Reviewed in http://llvm-reviews.chandlerc.com/D2917

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@204849 91177308-0d34-0410-b5e6-96231b3b80d8