libctf: properly handle ctf_add_type of forwards and self-reffing structs

The code to handle structures (and unions) that refer to themselves in
ctf_add_type is extremely dodgy.  It works by looking through the list
of not-yet-committed types for a structure with the same name as the
structure in question and assuming, if it finds it, that this must be a
reference to the same type.  This is a linear search that gets ever
slower as the dictionary grows, requiring you to call ctf_update at
intervals to keep performance tolerable: but if you do that, you run
into the problem that if a forward declared before the ctf_update is
changed to a structure afterwards, ctf_update explodes.

The last commit fixed most of this: this commit can use it, adding a new
ctf_add_processing hash that tracks source type IDs that are currently
being processed and uses it to avoid infinite recursion rather than the
dynamic type list: we split ctf_add_type into a ctf_add_type_internal,
so that ctf_add_type itself can become a wrapper that empties out this
being-processed hash once the entire recursive type addition is over.
Structure additions themselves avoid adding their dependent types
quite so much by checking the type mapping and avoiding re-adding types
we already know we have added.

We also add support for adding forwards to dictionaries that already
contain the thing they are a forward to: we just silently return the
original type.

v4: return existing struct/union/enum types properly, rather than using
    an uninitialized variable: shrinks sizes of CTF sections back down
    to roughly where they were in v1/v2 of this patch series.
v5: fix tabdamage.

libctf/
	* ctf-impl.h (ctf_file_t) <ctf_add_processing>: New.
	* ctf-open.c (ctf_file_close): Free it.
	* ctf-create.c (ctf_serialize): Adjust.
	(membcmp): When reporting a conflict due to an error, report the
	error.
	(ctf_add_type): Turn into a ctf_add_processing wrapper.  Rename to...
	(ctf_add_type_internal): ... this.  Hand back types we are already
	in the middle of adding immediately.  Hand back structs/unions with
	the same number of members immediately.  Do not walk the dynamic
	list.  Call ctf_add_type_internal, not ctf_add_type.  Handle
	forwards promoted to other types and the inverse case identically.
	Add structs to the mapping as soon as we intern them, before they
	gain any members.

libctf/ctf-impl.h

@@ -294,6 +294,7 @@ struct ctf_file
   /* Allow the caller to Change the name of link archive members.  */
   ctf_link_memb_name_changer_f *ctf_link_memb_name_changer;
   void *ctf_link_memb_name_changer_arg; /* Argument for it.  */
+  ctf_dynhash_t *ctf_add_processing; /* Types ctf_add_type is working on now.  */
   char *ctf_tmp_typeslice;	  /* Storage for slicing up type names.  */
   size_t ctf_tmp_typeslicelen;	  /* Size of the typeslice.  */
   void *ctf_specific;		  /* Data for ctf_get/setspecific().  */