Remove c++14-conditional constexpr

[smessmer]

Stack from ghstack:

• Remove c++14-conditional constexpr #30916 Remove c++14-conditional constexpr

These macros said "make it constexpr if we're in C++14". Since we're now always C++14, we can just say "constexpr" isntead.

Differential Revision: D18869635

[kostmo]

💊 CircleCI build failures summary and remediations

As of commit d55783f:

None of the build failures appear to be your fault.

• 1/1 broken upstream at merge base 22044c6 since Jan 07

  You may want to rebase on the viable/strict branch (expand for instructions)

  If your commit is newer than viable/strict, you can try basing on an older, stable commit:

  git fetch origin viable/strict
  git rebase --onto viable/strict $(git merge-base origin/master HEAD)
  

  If your commit is older than viable/strict:

  git fetch origin viable/strict
  git rebase viable/strict
  

  Check out the recency history of this "viable master" tracking branch.

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[facebook-github-bot]

This pull request has been merged in c21f899.