[C++] Warning about reference target not being found for templated types

Describe the bug

My documentation contains the following line (*):

.. cpp:var:: xbt::signal< void(const Actor &, const Host &previous_location)> Actor::on_host_change

This produces the following warning:

docstring of Actor::on_host_change:3: WARNING: cpp:identifier reference target not found: xbt::signal<void(const Actor&, const Host &previous_location)>

(*) note that this line does not actually appear in my source, but it's generated by an extension of mine, see below

To Reproduce
Steps to reproduce the behavior:

$ git clone https://framagit.org/simgrid/simgrid.git
$ cd simgrid
$ pip install -r docs/requirements.txt
$ cd docs
$ ./Build.sh

Expected behavior
I think that this warning should not be produced, as we don't want to document every variation of a given template. To me, this is very similar to #3320, but this time for the cpp domain.

Environment info

• OS: Linux
• Python version: 3.9.1+
• Sphinx version: 3.4.3-1 (from Debian)
• Sphinx extensions: docs/sources/_ext/autodoxy.py in the simgrid project is generating the cpp:var line from doxygen XML files

Run the reproduction from docs/requirements.txt and docs/Build.sh, using docs/sources/_ext/autodoxy.py as the source of the generated cpp:var directive. Trace how the C++ domain resolves the templated reference and determine how to avoid the warning for this case. Done means the documented build no longer emits the missing-reference warning.