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

[jloser]

Summary

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.

libcxx/include/filesystem

operator>>(basic_istream<_CharT, _Traits>& __is, path& __p) {
     basic_string<_CharT, _Traits> __tmp;
     __is >> __quoted(__tmp);
     __p = __tmp;
     return __is;
   }
 #endif // !_LIBCPP_HAS_NO_LOCALIZATION
 
   friend _LIBCPP_INLINE_VISIBILITY bool operator==(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) == 0;
+    return __lhs.__compare(__rhs.__pn_) == 0;
   }
   friend _LIBCPP_INLINE_VISIBILITY bool operator!=(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) != 0;
+    return __lhs.__compare(__rhs.__pn_) != 0;
   }
   friend _LIBCPP_INLINE_VISIBILITY bool operator<(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) < 0;
+    return __lhs.__compare(__rhs.__pn_) < 0;
   }
   friend _LIBCPP_INLINE_VISIBILITY bool operator<=(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) <= 0;
+    return __lhs.__compare(__rhs.__pn_) <= 0;
   }
   friend _LIBCPP_INLINE_VISIBILITY bool operator>(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) > 0;
+    return __lhs.__compare(__rhs.__pn_) > 0;
   }
   friend _LIBCPP_INLINE_VISIBILITY bool operator>=(const path& __lhs, const path& __rhs) noexcept {
-    return __lhs.compare(__rhs) >= 0;
+    return __lhs.__compare(__rhs.__pn_) >= 0;
   }
 
   friend _LIBCPP_INLINE_VISIBILITY path operator/(const path& __lhs,
                                                   const path& __rhs) {
     path __result(__lhs);
     __result /= __rhs;
     return __result;
   }