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/unsigned-struct.ispc

@@ -1,4 +1,4 @@
-// "unsigned" qualifier is illegal with "uniform struct Foo" typ
+// "unsigned" qualifier is illegal with "varying struct Foo" typ
 
 struct Foo {
     float x;