[libcxx][ranges] Fix `ranges::empty` when begin, end, and empty members are provided.

Before this commit, we'd get a compilation error because the operator() overload was ambiguous.

Differential Revision: https://reviews.llvm.org/D102263

GitOrigin-RevId: 98e4fd0701d0c65229db876ce338c723231d8cb4
2 files changed