[libc++] Adds C++26 support.

Clang has been updated to support C++26, this adds the same support for
libc++. At the moment C++23 and C++26 are identical. During the next
plenary in June the first C++26 papers will be voted on.

Note like Clang this patch uses C++26 is the internal part and C++2c in
the user visible part.

Depends on D150795

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D151026