Add static_assert to is_sorted() for the lookup tables in validate.cpp

[bhardwajs]

Description

With VS 2019 16.6 Preview 1 available, shipping toolset supports constexpr is_sorted() which means we can static_assert is_sorted in validate.cpp. This PR makes that change and closes #455.

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[cbezault]

Thanks for the contribution!

[StephanTLavavej]

FYI @bhardwajs - 21944c1 is listed as "bhardwajs and Sumit Bhardwaj committed yesterday", indicating that some of your commits are tagged with an email address that GitHub isn't aware of. You can teach GitHub about alternate email addresses by clicking on your avatar in the upper right > Settings > Emails > Add email address. Then such commits will be properly associated with your GitHub account.