[libc++] Implements concept default_initializable.

Implements:
- LWG3149 DefaultConstructible should require default initialization

Implements parts of:
 - P0898R3 Standard Library Concepts
 - P1754 Rename concepts to standard_case for C++20, while we still can

Depends on D91986

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D93461