Update defaults for variability of pointed-to types.

Now, if rate qualifiers aren't used to specify otherwise, varying
pointers point to uniform types by default.  As before, uniform
pointers point to varying types by default.

   float *foo;  // varying pointer to uniform float
   float * uniform foo;  // uniform pointer to varying float

These defaults seem to require the least amount of explicit
uniform/varying qualifiers for most common cases, though TBD if it
would be easier to have a single rule that e.g. the pointed-to type
is always uniform by default.

tests/ptr-8.ispc

@@ -1,13 +1,13 @@
 
 export uniform int width() { return programCount; }
 
-void inc(int * uniform v) {
+void inc(varying int * uniform v) {
     ++*v;
 }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     int a = aFOO[programIndex];
-    int * uniform b = &a;
+    varying int * uniform b = &a;
     if (a <= 2)
         inc(b);
     RET[programIndex] = a;