Implement unbound varibility for struct types.

Now, if a struct member has an explicit 'uniform' or 'varying'
qualifier, then that member has that variability, regardless of
the variability of the struct's variability.  Members without
'uniform' or 'varying' have unbound variability, and in turn
inherit the variability of the struct.

As a result of this, now structs can properly be 'varying' by default,
just like all the other types, while still having sensible semantics.

tests_errors/new-delete-6.ispc

@@ -1,4 +1,4 @@
-// Can't convert from type "uniform int32 * varying" to type "uniform int32 * uniform" for return
+// Can't convert from type "uniform int32 * varying" to type "varying int32 * uniform" for return
 
 int * uniform func(int x) {
     return new int[x];