[libc++] Implement `operator<=>` for `shared_ptr`

Implements part of:

* P1614R2 The Mothership has Landed

Fixes LWG3427

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D130852

GitOrigin-RevId: 3b217f2f127ebf5da5b87485dd9bc33ae1a66427
8 files changed