[PATCH 00/12] remove some cleanups using a cleanup function

[Tom Tromey <tom at tromey dot com>]

This is the series to introduce a new cleanup_function class, that can
be used in situations where a simple function call is needed to do
some cleanup.

As I mentioned in the other thread, I was (and still am) uncertain as
to whether this is a good idea.  My worry is just that this will be
used in ways leading to unclear code.  (You can judge for yourself
whether this series has already entered that territory...)

A couple of rules I used when deciding when to use the new class:

* There should not be too many uses of the existing cleanup (one or
  two).  If there are more than several it should probably be a
  bespoke class.

* Any lambdas should capture only a small number of things (one or
  maybe two), and only capture by value should be used.

Hopefully I actually followed these.

I wonder if the new class should take a std::function rather than a
gdb::function_view.  Some of the changes needs a bit of extra code to
deal with the latter.

Also I wonder if it should be possible to default-construct a
cleanup_function, and then set it later.  This would eliminate some
uses of gdb::optional; and seemed maybe reasonable given that the
class already has to maintain a (sort of-) validity flag.

This series includes a couple of cleanup-related comment fixes for
things I happened to notice along the way.

Regression tested by the buildbot.

Tom