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_errors/deref-4.ispc

@@ -1,4 +1,4 @@
-// Illegal to dereference non-pointer or reference type "float"
+// Illegal to dereference non-pointer or reference type "varying float"
 
 float func(float a) {
     *a = 0;