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

Implements part of:

  - P1614R2 The Mothership has Landed

Fixes LWG3426

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D130838

GitOrigin-RevId: 735240b38fd7a249474a66f96cb98587658010af
9 files changed