The reaction of ${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2{{}^{+}\mathrm{Cl}}^{-}$ with $\mathrm{CuCl}$ gives

This is Sandmeyer's reaction. In this reaction, aryl halides are formed from diazonium salts. It is a type of nucleophilic aromatic substitution reaction.

${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2{{}^{+}\mathrm{Cl}}^{-}+\mathrm{CuCl}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}$

So, chlorobenzene is the final product.