[clang-tools-extra] [clang-tidy] add modernize-use-constexpr check (PR #162741)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

Can you add more test cases with anonymous union members and constructors?

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I realise that suggesting `llvm::any_of` or `llvm::all_of` on short `for` loops is my personal preference. It is not listed in llvm coding standards. Please ignore these suggestions if you don't like them.

https://github.com/llvm/llvm-project/pull/162741