[llvm] [libc++] Implement ranges::contains (PR #65148)

[Konstantin Varlamov via llvm-commits]

================
@@ -0,0 +1,252 @@
+//===----------------------------------------------------------------------===//
----------------
var-const wrote:

Can you please go through various `robust` test files and add tests for `contains` where it makes sense? Let me know if you need any help with that!

https://github.com/llvm/llvm-project/pull/65148