[Refactoring] Use `FieldTypeTuple!S` instead of `typeof(S.tupleof)`.

`FieldTypeTuple` is what have to be used when searching through fields as it ignores hidden fields which we don't need here.

Replace in `std.traits.hasElaborate{CopyConstructor,Assign,Destructor}`. This is a refactoring change as hidden fields didn't affect these templates behaviour.
Merge pull request #1262 from denis-sh/use-`FieldTypeTuple!S`-instead-of-`typeof(S.tupleof)`

Use `FieldTypeTuple!S` instead of `typeof(S.tupleof)`.