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-22.ispc

@@ -6,14 +6,14 @@ struct Foo {
     uniform float b;
 };
 
-void update(Foo * varying fp) {
+void update(uniform Foo * varying fp) {
     ++fp;
     fp->a -= 1;
     fp->b = 1;
 }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    Foo f[2] = { { 1234, 4321 }, { aFOO[programIndex], 5 } };
+    uniform Foo f[2] = { { 1234, 4321 }, { aFOO[programIndex], 5 } };
     update(f);
     RET[programIndex] = f[1].a;
 }