[libc++] Implement P2499R0 (`string_view` range constructor should be `explicit`)

Reviewed By: #libc, philnik, Mordante

Spies: Mordante, jloser, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D130785

GitOrigin-RevId: 8be1197285bdff6e03bf26b77052daa710f73068