ir: validate field types on field creation

ir: validate field types on field creation

This patch removes the validation that is done when adding
fields to structure/variant field types, and when creating
array/sequence field types with specific element types. This
is needed because a variant with an unset tag type may be
created and thus exist in any of those compound types. This
variant will be resolved when the time comes.

A valid field type is still required when creating a concrete
field out of it and when serializing it.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>