Things which dissolve are called solutes and the liquid in which they dissolve is called a solvent to form a solution.
During the process of freezing, only physical state changes but the substance going through this change is chemically the same, water.
And the rate of formation of ice is equal to the rate of formation of water at the freezing point.
So at any point of time, the concentration of ice and water remains same; at the freezing point.
So we cant assign anyone as solute and solvent as such. Thus, both are taken to be solvent coz it's the same substance essentially.