[clang] [Clang] Add additional tests for constexpr initialization (PR #181965)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

I'm not sure we need to remove the other test file as a part of this commit, otherwise the tests lgtm

https://github.com/llvm/llvm-project/pull/181965