[PATCH 00/30] Rewrite the DWARF "partial" reader

[Tom Tromey <tom@tromey.com>]

This series removes the DWARF partial symbol reader, replacing it with
a new indexer which is significantly -- about an order of magnitude --
faster.

The new indexer is roughly based on the DWARF .debug_names approach.
That is, rather than construct full names for all interesting DIEs,
it instead records the name given in the DWARF.

Note, though, that this series does not actually update gdb's
.debug_names writer to be correct.  The new index is constructed so
that this should be easy to do, but the series is long enough, and
complicated enough, that I did not want to add this on.  (It's not
enough to update the writer -- the reader must also be updated, and
one must decide how much backward compatibility to keep.)

Why write a new indexer, rather than simply fix the psymtab reader?  I
have tried that multiple times over the years, without success.  I
believe that parallelism is the key to increased performance, and this
is just too hard to retrofit onto the existing reader.

This series starts with a number of refactorings.  I've already landed
a number of these, but the ones that remain are specific to this
series, so I didn't try to submit them separately.

The new code is also broken into separate patches for simpler review.

Some of the performance gains are actually just an illusion -- some
work is moved to the background, and if the main part of gdb needs the
information, it will once again appear to be slow.  However, this is
still an improvement for some common situations, for example simply
starting gdb on a large program.  My preferred long-term fix here
would be to replace DWARF with something that's less terrible for
reading; but failing that we can maybe eke out some small incremental
gains through further micro-optimization, or by farming out this
scanning to something like debuginfod.

I've regression tested each patch in this series.  Only the patch to
enable the new indexer is intended to affect operation at all.

Tom