[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/epsilon.pass.cpp

@@ -44,6 +44,10 @@ int main()
     test<unsigned long>(0);
     test<long long>(0);
     test<unsigned long long>(0);
+#ifndef _LIBCPP_HAS_NO_INT128
+    test<__int128_t>(0);
+    test<__uint128_t>(0);
+#endif
     test<float>(FLT_EPSILON);
     test<double>(DBL_EPSILON);
     test<long double>(LDBL_EPSILON);