Modify rules for default variability of pointed-to types.

Now, the pointed-to type is always uniform by default (if an explicit
rate qualifier isn't provided).  This rule is easier to remember and
seems to work well in more cases than the previous rule from 6d7ff7eba2.

tests_errors/new-delete-6.ispc

@@ -1,4 +1,4 @@
-// Can't convert from type "uniform int32 * varying" to type "varying int32 * uniform" for return
+// Can't convert from type "uniform int32 * varying" to type "uniform int32 * uniform" for return
 
 int * uniform func(int x) {
     return new int[x];