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/struct-uniform-gather-3.ispc

@@ -0,0 +1,7 @@
+// Gather operation is impossible due to the presence of struct member "x" with uniform type
+
+struct Foo { uniform int x; };
+
+void a(uniform Foo * varying array, int index) {
+    Foo v = array[index];
+}