[libc++][utility][NFC] Refactored safe integer `cmp_xxxx` functions to use the...

[libc++][utility][NFC] Refactored safe integer `cmp_xxxx` functions to use the `__libcpp_is_integer` concept (#78115)

Replaced a functionally identical internal concept helper.

References:
- https://eel.is/c++draft/utility.intcmp
- https://eel.is/c++draft/basic.fundamental



Co-authored-by: Zingam <zingam@outlook.com>