Each plugin currently uses its own recursive AST run, which adds up.
This patch adds another shared plugin which internally contains all
(suitable) plugins and dispatches to them from the same one recursive
run. This patch converts ~25 plugins and for starmath's accessibility.cxx
reduces clang build time from 5.43s to 5.14s (and it's 4.39s without any
plugins). As there are almost 50 more plugins to go, this can theoretically
result in 4.56s final time, although probably not all plugins can be
that easily converted, if at all.

This mostly requires very little change in many plugins (see e.g.
BadStatics), some even work without any functionality change (e.g.
CharRightShift). Traverse* calls require some changes but are often
not that difficult. WalkUp* probably can't be supported, although some
plugins can(?) possibly be adjusted to not rely on them. And of course
some plugins can be left as they are, using their own recursive run.
See description at the top of generator.cxx for description of how to
convert a plugin.

The sharedvisitor.cxx source is generated based on scanning relevant
plugin sources using a clang-based scanner/generator. The generated
source is intentionally included instead of getting always generated,
as the generating currently takes some time, so it should get updated
in git whenever a change in a plugin triggers a source change in it.

Change-Id: Ia0d2e3a5a464659503dbb4ed6c20b6cc89b4de01
Reviewed-on: https://gerrit.libreoffice.org/68026
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>