[clang] [clang-tools-extra] [libcxx] [llvm] [libc++] implement views::concat (PR #120920)

[Nikolas Klauser via llvm-commits]

https://github.com/philnik777 requested changes to this pull request.

Please make sure you're following the [libc++ Coding Guidelines](https://libcxx.llvm.org/CodingGuidelines.html).

https://github.com/llvm/llvm-project/pull/120920