[clang] [libcxx] [libc++][Clang] Added explanation why is_constructible evaluated to false. Updated the diagnostics checks in libc++ tests. (PR #144220)

[Shamshura Egor via cfe-commits]

egorshamshura wrote:

> Can you update the title/description to reflect the entire change (ie reuse the title/description of the previous PR)? Otherwise LGTM
> 
> Thanks for fixing the libc++ tests

I have changed title and added description. How can I rerun job?

https://github.com/llvm/llvm-project/pull/144220