[libc++] [P0935] [C++20] Eradicating unnecessarily explicit default...

[libc++] [P0935] [C++20] Eradicating unnecessarily explicit default constructors from the standard library.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0935r0.html

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D91292