[clang] [clang-tools-extra] [libcxx] [llvm] [libc++][ranges] P2542R8: Implement `views::concat` (PR #120920)

[Nhat Nguyen via cfe-commits]

changkhothuychung wrote:

> > @frederick-vs-ja just follow up on this, is there anything else I need to address for this PR?
> 
> It looks like you are missing the Release Notes entry, FTM is not set, etc. house keeping stuff. Please check some other complete PRs for what else you need to do.

What's FTM? Can you give me an example PR that has those? 

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