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

[Nhat Nguyen via cfe-commits]

changkhothuychung wrote:

> @changkhothuychung The be more specific. You should add the paper entry here:
> 
> https://github.com/llvm/llvm-project/blob/4887e41055686eede9c155e6b3296b92fe86c2d5/libcxx/docs/ReleaseNotes/21.rst?plain=1#L41
> 
> 
> And update the status accordingly:
> https://github.com/llvm/llvm-project/blob/4887e41055686eede9c155e6b3296b92fe86c2d5/libcxx/docs/Status/Cxx2cPapers.csv?plain=1#L57
> 
> It also looks like there are additional related LWGs that you should implement probably :
> 
> https://github.com/llvm/llvm-project/blob/4887e41055686eede9c155e6b3296b92fe86c2d5/libcxx/docs/Status/Cxx2cIssues.csv?plain=1#L72

I think let's address LWG4082 in another PR for better tracking? 

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