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/unif-struct-test-123.ispc

@@ -3,7 +3,7 @@ export uniform int width() { return programCount; }
 
 
 struct Foo {
-    float x;
+    varying float x;
 };
 void bar(uniform struct Foo * uniform f) { ++((*f).x); }
 export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {