Temperature dependence of molarity and molality
Recall that molarity $c$ is

$$c=\frac{n_\text{solute}}{V_\text{solution}}$$

whereas for molality $b$ is

$$b=\frac{n_\text{solute}}{m_\text{solvent}}$$

While in the extreme case you mentioned, both quantities are not defined because all the solvent is either boiled away (or for heat-sensitive species, the solvent can even decompose under heating), molarity depends on volume, and the volume of the solution depends on temperature due to expansion/contraction.