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
Mark de Wever