.github: Add missing mappings implied by owning driver sources

Author: AnthonyLatsis

.github/CODEOWNERS

@@ -270,6 +270,7 @@
 /utils/vim/                                                   @compnerd
 
 # validation-test
+/validation-test/Driver/                  @artemcm
 /validation-test/IDE/                     @ahoppen @bnbarham @rintaro @hamishknight
 /validation-test/IRGen/                   @rjmccall
 /validation-test/Parse/                   @ahoppen @bnbarham @CodaFi @DougGregor @hamishknight @rintaro