c-family: Use -Wdiscarded-qualifiers for ignored qualifiers in __atom…

…ic_*

This matches other compiler diagnostics.  No test updates are needed
because c-c++-common/pr95378.c does not match a specific -W option.

Fixes commit d2384b7 ("c-family:
check qualifiers of arguments to __atomic built-ins (PR 95378)").

gcc/c-family/

	PR c/113050
	* c-common.cc (get_atomic_generic_size): Use
	OPT_Wdiscarded_qualifiers instead of
	OPT_Wincompatible_pointer_types.

gcc/c-family/c-common.cc

@@ -7471,7 +7471,7 @@ get_atomic_generic_size (location_t loc, tree function,
 		return 0;
 	      }
 	    else
-	      pedwarn (loc, OPT_Wincompatible_pointer_types, "argument %d "
+	      pedwarn (loc, OPT_Wdiscarded_qualifiers, "argument %d "
 		       "of %qE discards %<const%> qualifier", x + 1,
 		       function);
 	  }
@@ -7485,7 +7485,7 @@ get_atomic_generic_size (location_t loc, tree function,
 		return 0;
 	      }
 	    else
-	      pedwarn (loc, OPT_Wincompatible_pointer_types, "argument %d "
+	      pedwarn (loc, OPT_Wdiscarded_qualifiers, "argument %d "
 		       "of %qE discards %<volatile%> qualifier", x + 1,
 		       function);
 	  }