Sodium chloride + sulfuric acid
How is $\ce{HCl}$, which is not even a reactant, 'displaced' from
  $\ce{NaCl}$, as there is no $\ce{HCl}$ to be displaced?


Well, there will be some $\ce{HCl}$ due to a well-known chemical reaction which is used both in the lab as well as in production to get the hydrogen chloride: 

$$\ce{NaCl(s) + H2SO4(s) → NaHSO4(s) + HCl(g)}$$

The reaction proceeds at room temperature, but note, that the reagents should be dry. If you heat it up above 200 °C reaction proceeds even further:

$$\ce{NaCl(s) + NaHSO4(s) → HCl(g) + Na2SO4(s)}$$

Hydrochloric acid is then prepared by dissolving hydrogen chloride in water.