ir: copy variants and sequences when setting a field path

ir: copy variants and sequences when setting a field path

Variants and sequences might be used in multiple structures. Therefore,
they are replaced by a copy with the path to their tag or sequence
length field set.

This is done on freeze, hence a user should not expect to be able
to modify a sequence's or variant's attributes subsequently.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>