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-25.ispc

@@ -1,7 +1,7 @@
 
 export uniform int width() { return programCount; }
 
-void update(float<2> * varying vp) {
+void update(varying float<2> * vp) {
     vp->y = 0;
 }