libctf: add a deduplicator-specific type mapping table

When CTF linking is done, the linker has to track the association
between types in the inputs and types in the outputs.  The deduplicator
does this via the cd_output_emission_hashes, which maps from hashes of
types (valid in both the input and output) to the IDs of types in the
specific dict in which the cd_emission_hashes is held.  However, the
nondeduplicating linker and ctf_add_type used a different mechanism, a
dedicated hashtab stored in the ctf_link_type_mapping, populated via
ctf_add_type_mapping and queried via the ctf_type_mapping function.  To
allow the same functions to be used for variable and symbol population
in both the deduplicating and nondeduplicating linker, the deduplicator
carefully transferred all its input->output mappings into this hashtab
before returning.

This is *expensive*. The number of entries in this hashtab scales as the
number of input types, and unlike the hashing machinery the type mapping
machinery (the only other thing which scales that way) has not been much
optimized.

Now the nondeduplicating linker is gone, we can throw this out, move
the existing type mapping machinery to ctf-create.c and dedicate it to
ctf_add_type alone, and add a new function ctf_dedup_type_mapping which
uses the deduplicator's built-in knowledge of type mappings directly,
without requiring an expensive repopulation phase.

This speeds up a test link of nouveau.ko (a good worst-case candidate
with a lot of types in each of a lot of input files) from 9.11s to 7.15s
in my testing, a speedup of over 20%.

libctf/ChangeLog
2021-03-02  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-impl.h (ctf_dict_t) <ctf_link_type_mapping>: No longer used
	by the nondeduplicating linker.
	(ctf_add_type_mapping): Removed, now static.
	(ctf_type_mapping): Likewise.
	(ctf_dedup_type_mapping): New.
	(ctf_dedup_t) <cd_input_nums>: New.
	* ctf-dedup.c (ctf_dedup_init): Populate it.
	(ctf_dedup_fini): Free it again.  Emphasise that this has to be
	the last thing called.
	(ctf_dedup): Populate it.
	(ctf_dedup_populate_type_mapping): Removed.
	(ctf_dedup_populate_type_mappings): Likewise.
	(ctf_dedup_emit): No longer call it.  No longer call
	ctf_dedup_fini either.
	(ctf_dedup_type_mapping): New.
	* ctf-link.c (ctf_unnamed_cuname): New.
	(ctf_create_per_cu): Arguments must be non-null now.
	(ctf_in_member_cb_arg): Removed.
	(ctf_link): No longer populate it.  No longer discard the
	mapping table.
	(ctf_link_deduplicating_one_symtypetab): Use
	ctf_dedup_type_mapping, not ctf_type_mapping.  Use
	ctf_unnamed_cuname.
	(ctf_link_one_variable): Likewise.  Pass in args individually: no
	longer a ctf_variable_iter callback.
	(empty_link_type_mapping): Removed.
	(ctf_link_deduplicating_variables): Use ctf_variable_next, not
	ctf_variable_iter.  No longer pack arguments to
	ctf_link_one_variable into a struct.
	(ctf_link_deduplicating_per_cu): Call ctf_dedup_fini once
	all link phases are done.
	(ctf_link_deduplicating): Likewise.
	(ctf_link_intern_extern_string): Improve comment.
	(ctf_add_type_mapping): Migrate...
	(ctf_type_mapping): ... these functions...
	* ctf-create.c (ctf_add_type_mapping): ... here...
	(ctf_type_mapping): ... and make static, for the sole use of
	ctf_add_type.

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