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/initexpr-2.ispc

@@ -1,3 +1,3 @@
-// Initializer list for array "int32[2][4]" must have no more than 2 elements (has 3)
+// Initializer list for array "varying int32[2][4]" must have no more than 2 elements (has 3)
 
 int a[2][4] = { { 1, 2, 3 }, { 1, 2, 3, 4 }, 1 };