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/struct-gather-3.ispc

@@ -7,13 +7,13 @@ struct Foo {
     float f;
 };
 
-float func(uniform Foo foo[], int offset) {
+float func(Foo foo[], int offset) {
     return foo[offset].f;
 }
 
 export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     float a = aFOO[programIndex];
-    uniform Foo foo[17];
+    Foo foo[17];
     uniform int i;
     for (i = 0; i < 17; ++i)
         foo[i].f = i*a;