ctf: speed up the dwarf2ctf duplicate detector some more

ctf: speed up the dwarf2ctf duplicate detector some more

The duplicate detector's alias detection pass is still doing unnecessary
work.  When a non-opaque structure is marked shared, it is sometimes
necessary to do another deduplication scan, because the marking may have
marked types used within the structure as shared, which may require yet
more types (e.g. opaque uses of of that type in other modules) to be
marked shared as well.  So while we know this pass can only affect
structures/ unions/enums that have names, and their interior types, it
would seem that we must keep scanning them to see if they need
deduplication until none remain.

However, there is one exception: if a non-opaque type and its
corresponding opaque type are both already marked shared, or if we have
just processed them and marked them accordingly, we know that we will
never need to re-mark those particular types again, since they can't be
more shared than they already are: so we can remove them from
consideration in future passes.  Because we are only opening DWARF files
in this pass as needed now, this hugely cuts down the number of files we
process in subsequent passes: we still see the same number of passes,
but passes after the first (which marks tens of thousands of opaque
types as shared) only open a few files, mark a few hundred types, and
flash past in under a second.

In my tests, the alias fixup pass now takes under 10s, which can be
more or less ignored: all other passes other than initialization
and writeout are much more expensive.  (Before this series, it took
over a minute on the fastest machine I have access to, and over three
minutes on SPARC.)

Orabug: 25815306
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: tomas.jedlicka@oracle.com