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/assign-struct-with-const-member.ispc

@@ -1,4 +1,4 @@
-// Illegal to assign to type "uniform struct Foo" due to element "a" with type "const varying int32"
+// Illegal to assign to type "varying struct Foo" due to element "a" with type "const varying int32"
 
 struct Foo {
     const int a;