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/struct-nested-4.ispc

@@ -17,7 +17,7 @@ void init(uniform Ray rays[], uniform int count, float v) {
     }
 }
 
-void zero_dx(Ray * uniform r) {
+void zero_dx(varying Ray * uniform r) {
     r->d.x = 0;
 }