What is the reaction of concentrated HCl and CuCl2?

The copper chloride will just dissolve in the hydrochloric acid because even concentrated hydrochloric acid is over 50% water and copper chloride is soluble in water. There is no chemical reaction, just dissolving a salt in water.

Well it depends on the concentration of the HCl.

At low concentrations, HCl completely ionises as it reacts with water to form aqueous H[math]_{3}[/math]O[math]^{+}[/math] and Cl- ions;-

HCl(g) + H[math]_{2}[/math]O(l) → H[math]_{3}[/math]O[math]^{+}[/math](aq) + Cl[math]^{-}[/math](aq)

CuCl[math]_{2}[/math] also ionises in aqueous solution to form Cu+2 and Cl- ions;-

CuCl[math]_{2}[/math](s) → Cu[math]^{+2}[/math](aq) + 2 Cl[math]^{-}[/math](aq)

However, at very high concentrations of HCl, a significant amount of un-ionised HCl is present. This reacts with dissolved Cu[math]^{+2}[/math] ions to form the complex anion; tetrachloridocuprate(-2) (CuCl[math]_{4}^{-2}[/math]);-

Cu[math]^{+2}[/math](aq) + 4 HCl → CuCl[math]_{4}^{-2}[/math] + 4H[math]^{+}[/math](aq)