[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/digits.pass.cpp

@@ -43,6 +43,10 @@ int main()
     test<unsigned long, sizeof(long) == 4 ? 32 : 64>();
     test<long long, 63>();
     test<unsigned long long, 64>();
+#ifndef _LIBCPP_HAS_NO_INT128
+    test<__int128_t, 127>();
+    test<__uint128_t, 128>();
+#endif
     test<float, FLT_MANT_DIG>();
     test<double, DBL_MANT_DIG>();
     test<long double, LDBL_MANT_DIG>();