[libc++] Updates C++2b to C++23.

During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.

This updates the reference to c++2b to c++23 and updates the __cplusplus
macro.

Note since we use clang-tidy 16 a small work-around is needed. Clang
knows -std=c++23 but clang-tidy not so for now force the lit compiler
flag to use -std=c++2b instead of -std=c++23.

Reviewed By: #libc, philnik, jloser, ldionne

Differential Revision: https://reviews.llvm.org/D150795

libcxx/test/std/strings/basic.string/string.cons/nullptr.compile.pass.cpp

@@ -10,8 +10,8 @@
 
 // <string>
 
-// basic_string(nullptr_t) = delete; // C++2b
-// basic_string& operator=(nullptr_t) = delete; // C++2b
+// basic_string(nullptr_t) = delete; // C++23
+// basic_string& operator=(nullptr_t) = delete; // C++23
 
 #include <string>
 #include <type_traits>