Many monsters have the following ability:
Legendary Resistance (x/day). If the [monster] fails a saving throw, it can choose to succeed instead.
This ability allows the monster to pass any saving throw, regardless of DC.
Player characters, on the other hand, have many abilities which allow them to increase their likelihood of passing a saving throw: examples include Indomitable, Dark One's Blessing, and so forth. To my knowledge, however, none of these abilities provide a means of unconditionally passing a saving throw.
Besides using an effect like Polymorph which replaces a player character's stat sheet with that of a monster, do player characters have any options which provide them with the ability to automatically pass a saving throw, regardless of its DC?