[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/has_infinity.pass.cpp

@@ -42,6 +42,10 @@ int main()
     test<unsigned long, false>();
     test<long long, false>();
     test<unsigned long long, false>();
+#ifndef _LIBCPP_HAS_NO_INT128
+    test<__int128_t, false>();
+    test<__uint128_t, false>();
+#endif
     test<float, true>();
     test<double, true>();
     test<long double, true>();