[llvm] [libcxx] [mlir] [libc++][concepts] Implements concept helper `__libcpp_integer` (PR #78086)

[Mark de Wever via llvm-commits]

https://github.com/mordante commented:

> @mordante @frederick-vs-ja This adds `std::__libcpp_integer` concept as a prerequisite from this discussion: [#77967 (comment)](https://github.com/llvm/llvm-project/pull/77967#discussion_r1451501514)
> 
>     * Any ideas for an alternative name, e.g.
>       
>       * `std::__libcpp_true_integer`
>       * `std::__libcpp_integer_type`
> 
>     * I have not replaced `__is_safe_integral_cmp` in this patch as suggested [[libc++][numeric] P0543R3: Saturation arithmetic #77967 (comment)](https://github.com/llvm/llvm-project/pull/77967#discussion_r1451099830). I can do it in a follow-up patch.

Thanks. I would keep the name as is. Why do you want to provide different names.

LGTM modulo some small nits.

https://github.com/llvm/llvm-project/pull/78086