This is information that the compiler already has, and should be exposed
so that the library doesn't need to reimplement the exact same
functionality.
- __is_nullptr
 - __is_bounded_array
 - __is_scoped_enum
 - __is_unbounded_array
 - __is_copy_constructible
 - __is_move_constructible
 - __is_copy_assignable
 - __is_move_assignable
 - __is_referenceable
 
Is a VLA a bounded array? (We support those as an extension in C++, unfortunately.)