Fix numeric_limits<XXX>::is_modulo for signed arithmetic types. We were reporting true, for all arithmetic types, which is incorrect. Fix the tests which were wrong, too. This fixes PR#20158.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@214371 91177308-0d34-0410-b5e6-96231b3b80d8