gdbsupport: add async parallel_for_each version

gdbsupport: add async parallel_for_each version

I would like to use gdb::parallel_for_each to implement the parallelism
of the DWARF unit indexing.  However, the existing implementation of
gdb::parallel_for_each is blocking, which doesn't work with the model
used by the DWARF indexer, which is asynchronous and callback-based.
Add an asynchronouys version of gdb::parallel_for_each that will be
suitable for this task.

This new version accepts a callback that is invoked when the parallel
for each is complete.

This function uses the same strategy as gdb::task_group to invoke the
"done" callback: worker threads have a shared_ptr reference to some
object.  The last worker thread to drop its reference causes the object
to be deleted, which invokes the callback.

Unlike for the sync version of gdb::parallel_for_each, it's not possible
to keep any state in the calling thread's stack, because that disappears
immediately after starting the workers.  So all the state is kept in
that same shared object.

There is a limitation that the sync version doesn't have, regarding the
arguments you can pass to the worker objects: it's not possibly to rely
on references.  There are more details in a comment in the code.

It would be possible to implement the sync version of
gdb::parallel_for_each on top of the async version, but I decided not to
do it to avoid the unnecessary dynamic allocation of the shared object,
and to avoid adding the limitations on passing references I mentioned
just above.  But if we judge that it would be an acceptable cost to
avoid the duplication, we could do it.

Add a self test for the new function.

Change-Id: I6173defb1e09856d137c1aa05ad51cbf521ea0b0
Approved-By: Tom Tromey <tom@tromey.com>