[libc++][numeric] P0543R3: Saturation arithmetic (#77967)

Implements: https://wg21.link/P0543R3
- https://eel.is/c++draft/numeric.sat

Additional references:
- Division: https://eel.is/c++draft/expr.mul#4
- Arithmetic conversions: https://eel.is/c++draft/expr.arith.conv#1
- Clang builtins:
https://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions

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

---------

Co-authored-by: Zingam <zingam@outlook.com>
Co-authored-by: Mark de Wever <zar-rpg@xs4all.nl>

libcxx/include/module.modulemap.in

@@ -1580,6 +1580,7 @@ module std_private_numeric_pstl_transform_reduce    [system] {
   export *
 }
 module std_private_numeric_reduce                   [system] { header "__numeric/reduce.h" }
+module std_private_numeric_saturation_arithmetic    [system] { header "__numeric/saturation_arithmetic.h" }
 module std_private_numeric_transform_exclusive_scan [system] { header "__numeric/transform_exclusive_scan.h" }
 module std_private_numeric_transform_inclusive_scan [system] { header "__numeric/transform_inclusive_scan.h" }
 module std_private_numeric_transform_reduce         [system] { header "__numeric/transform_reduce.h" }