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

[Julian Schmidt via cfe-commits]

5chmidti wrote:

I've cleaned things up and separated-out the C++-11-only rule-set into a new PR for a more incremental review and merge here: #162741

- dropped support for declarations of multiple variables in one statement
- hopefully addressed all outstanding review comments

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