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-14.ispc

@@ -3,9 +3,9 @@ export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex];
-    float * uniform pa = &a;
-    int * uniform pb = (int *)pa;
-    float *uniform pc = (float *)pb;
+    varying float * uniform pa = &a;
+    varying int * uniform pb = (varying int *)pa;
+    varying float *uniform pc = (varying float *)pb;
     *pc = programIndex;
     RET[programIndex] = *pc;
 }