Fix for std::numeric_limits<std::_Signed128>::is_signed

[mtezych]

Hello,

I've noticed that the MSVC's STL provides std::numeric_limits template specialization
for the std::_Signed128 (a type which models the signed-integer-class concept) slightly incorrectly,
that is the std::numeric_limits<std::_Signed128>::is_signed is false instead of true:

• https://godbolt.org/z/EcbE7Mrnq

This change fixes that issue and adds a couple of compile-time checks
verifying that the std::numeric_limits<std::_Signed128> is properly specialized.

[CaseyCarter]

Thanks for fixing my mistake!

[strega-nil-ms]

Thanks for noticing!

[mtezych]

@microsoft-github-policy-service agree

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for fixing this bug, and congratulations on your first microsoft/STL commit! 🎉 😻 🚀

This will ship in VS 2022 17.6 Preview 2.