ctf: allow overriding of DIE attributes: use it for parent bias

ctf: allow overriding of DIE attributes: use it for parent bias

Currently, dwarf2ctf's CTF type emission machinery emits any given CTF
type based purely on the data in the corresponding DWARF DIE, with
possible adjustments based on the DIE's structural parent (useful for
structure members, if not so much for top-level types).  But from time
to time we need to adjust a CTF type depending on some property not of
its structural parent but of a type that depends on it: i.e., for
structures, we might want to adjust the offset of the members to cater
for the fact that this structure is being structurally merged with a
shorter assignment-compatible structure with identical name which
appeared in some translation unit that was processed earlier.

Currently, we handle this case, and only this case, by passing down a
'parent_bias' to all the CTF assembly functions.  Replace this with a
more generic mechanism whereby an array of 'overrides' can be passed
down to construct_ctf_id(), die_to_ctf(), and all subordinate assembly
functions: these overrides consist of an array of die_override_t's,
where each element can either override or add to the value of one DWARF
attribute: this kicks in for specific DWARF tags only. (Only numerical
attributes are supported, obviously.)

(We also pass the overrides down to type_id() so that overrides can
affect the ID of types and thus cause a single DWARF type to generate
multiple CTF types, though we do not use this facility in this commit.)

To process the attributes, we introduce a new private_find_override() to
search the override list, and private_dwarf_udata() to fetch a udata and
handle it. (We do not override dwarf_hasattr(): anything that wants to
override an attribute that may not exist has to call
private_find_override() itself.  If this happens a lot, we can introduce
an override for dwarf_hasattr() too.)

Currently we use this in exactly one place: in assemble_ctf_su_member(),
to replace the use of parent_bias.  Further uses will come in the next
commit: thanks to this commit, none of them will require adding new
parameters to all the CTF construction functions :)

Also rename the 'override' parameter on the CTF construction functions,
which was used by array assembly to indicate that CTF types should
replace their parent type, with a much less confusingly-named 'replace'
parameter.  (It was badly named before, but now that we have a parameter
named 'overrides' it is devastatingly badly named.)

Orabug: 25815129
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: tomas.jedlicka@oracle.com