[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

test/language.support/support.limits/limits/numeric.limits.members/quiet_NaN.pass.cpp

@@ -63,6 +63,10 @@ int main()
     test<unsigned long>();
     test<long long>();
     test<unsigned long long>();
+#ifndef _LIBCPP_HAS_NO_INT128
+    test<__int128_t>();
+    test<__uint128_t>();
+#endif
     test<float>();
     test<double>();
     test<long double>();