[libc++] Fix std::variant comparators not working on recursive instantiations (#182238)

We currently have an instantiation cycle in `variant`'s comparison
operators. This patch fixes that by replacing a `decltype(auto)` with an
explicit return type, removing the requirement to instantiate the
function in order to know the return type.

Fixes #182232

NOKEYCHECK=True
GitOrigin-RevId: 861b9cacdcda23cc1c610a2f5ab1f8533bb8e621