[clang-tools-extra] [clang-tidy] add modernize-use-constexpr check (PR #146553)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

> Gave this a very rought look, so only a couple of general questions/suggestions.
> 
> Could we do something with methods that repeat in some of the visitors, e.g. `bool TraverseType(QualType QT)` and others. Maybe macros..? I can't think of a better idea right now.

I will try to think of something

> Could we break this PR in multiple ones, e.g. first with support of CXX11, then CXX14 and so on? 1000-line `.cpp` file is hard to review. Also, with a PR for each standart we will understand better what changed and what new constexpr cases appear.

That's a valid point. I'll keep this as is for a while longer without the expectation that people will review it, so that I can resolve a few issues. I also want to fix my setup to be able to run on external code bases. Then I will split it up into {[11],[14, 17],[20],[23, 26]} for 4 reviews. I'm not splitting per version since the two pairs of versions are quite similar to each other. The PRs will probably be serialized and not stacked, because we all know how nice GitHubs PR stacking works.

https://github.com/llvm/llvm-project/pull/146553