libctf: create, types: conflicting types

The conflicting type kind is a CTF-specific prefix kind consisting purely of
an optional translation unit name.  It takes the place of the old hidden
bit: we have already seen it used to prefix types added with a
CTF_ADD_NONROOT flag.  The deduplicator will also use them to label
conflicting types from different TUs smushed into the same dict by the
CU-mapping mechanism: unlike the hidden bit, with this scheme users can tell
which CUs the conflicting types came from.

New API:

+int ctf_type_conflicting (ctf_dict_t *, ctf_id_t, const char **cuname);
+int ctf_set_conflicting (ctf_dict_t *, ctf_id_t, const char *);

(Frankly I expect ctf_set_conflicting to be used only by deduplicators and
things like that, but if we provide an option to query something we should
also provide an option to produce it...)

libctf/libctf.ver

@@ -77,6 +77,7 @@ LIBCTF_2.0 {
 	ctf_type_align;
 	ctf_type_kind;
 	ctf_type_kind_forwarded;
+	ctf_type_conflicting;
 	ctf_type_reference;
 	ctf_type_linkage;
 	ctf_type_pointer;
@@ -155,6 +156,7 @@ LIBCTF_2.0 {
 	ctf_add_member_bitfield;
 
 	ctf_set_array;
+	ctf_set_conflicting;
 
 	ctf_update;
 	ctf_discard;