[libc++] Avoid overload resolution in path comparison operators

Rework `std::filesystem::path::operator==` and friends to avoid overload
resolution and atomic constraint caching issues shown from
https://reviews.llvm.org/D113161.

Always call `__compare(string_view)` from the comparison operators which avoids
overload resolution.

Differential Revision: https://reviews.llvm.org/D114570