[llvm] [libc++] Implement ranges::contains_subrange (PR #66963)

[Konstantin Varlamov via llvm-commits]

var-const wrote:

> > Please make sure you add a description to your PR. This is what usually goes into the git log and we want those entries to be as descriptive and helpful for folks who read the git logs, thank you.
> 
> Do you have any specific suggestions what should be in the message? The libc++ "I implement something" patches are most of the time quite straight-forward, since you can look at the standard and know what's going on. It's also the best source, since it's the authoritative document here, and is also what anybody who cares to actually look at the patch should check against for correctness.

+1 -- I'm in favor of descriptive commit messages in general, but in this case I'm not sure what would be useful to add beyond "Implement ranges::contains_subrange".

https://github.com/llvm/llvm-project/pull/66963