WG21-P2407R5 Freestanding Library: Partial Classes

Feature-test macros (expected):

#define __cpp_lib_freestanding_algorithm   202311L
#define __cpp_lib_freestanding_array       202311L
#define __cpp_lib_freestanding_optional    202311L
#define __cpp_lib_freestanding_string_view 202311L
#define __cpp_lib_freestanding_variant     202311L

We're not entirely sure what our hosted implementation needs to do here. IF the only thing that we need to do is define these feature-test macros, then we'll accept a PR for this (like #3837) before we open up the floodgates to arbitrary C++26 features.