Per [range.view]/6, a view_interface isn't a base class of itself, so enable_view should report false. Also, current implementation strategy handles const but not volatile, IIUC cv-qualifiers should be consistent handled.

Coverage for (const) volatile types are added.

The original issue was reported as LLVM-132577, and the PR fixing it (LLVM-132582) was recently accepted for libc++.

Drive-by: Change the implement strategy of operator<=> for optional to be more consistent with the Standardese ([optional.syn]), as the old _Derived_from_specialization_of doesn't seem reusable.