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

[Samrudh Nelli via cfe-commits]

SamrudhNelli wrote:

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

Since @tbaederr was okay with removing the test file, and it felt like that was a duplication, I removed that file.
Do you mean to open a new PR to remove that file and discuss more about it?

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