libctf: create: addition of non-root types should not return root types

libctf: create: addition of non-root types should not return root types

If you add a non-root type to a dict, you should always get a new, unique
type ID back, even if a root-visible type with the same name already exists.
Unfortunately, if the root-visible type is a forward, and you're adding a
non-root-visible struct, union, or enum, the machinery to detect forwards
and promote them to the concrete type fires in this case and returns the
root-visible type!  If this is an enum being inserted hidden because its
enumerands conflict with some other enum, this will lead to failure later
on: in any case, it's seriously counterintuitive to add a non-root- visible
type and get a root-visible one instead.

Fix this by checking the root-visible flag properly and only checking for
forwards if this type is root-visible.  (This may lead to a certain degree
of proliferation of non-root-visible forwards: we can add a cleanup pass for
those later if needed.)

libctf/
	* ctf-create.c (ctf_add_sou_sized): Check the root-visible flag when
	doing forward promotion.
	(ctf_add_enum_internal): Likewise.
	(ctf_add_enum_encoded_internal): Likewise.