[libc++] Add C++20 stringstream::view()

Reviewed By: #libc, philnik, Mordante

Spies: Mordante, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D148641

GitOrigin-RevId: 49007a020c14a48062fac34c5c83c907d6ae1c31
7 files changed