(C++) Qualified names in signatures fail to generate cross-references

Describe the bug

If a C++ function parameter or return type contains a qualified name, then xref resolution fails by attempting to generate a link for the namespace name.

Given

.. cpp:namespace:: foo
.. cpp:class:: something 

.. cpp:function:: void bar(foo::something arg)

Expected: The foo::something parameter type generates a hyperlink to the something class defined within the foo namespace.

Actual: /.../repro/index.rst:8: WARNING: cpp:identifier reference target not found: foo [ref.identifier]

Using a qualified name in an inline reference (e.g. cpp:class or cpp:any) will resolve the object just fine. It appears that the generated signature nodes contain a reference to some foo (the namespace), which has no link target.

This usually will silently fail unless nitpicky mode is enabled.

How to Reproduce

An empty conf.py is sufficient. The following index.rst will fail to render when nitpicky-mode is enabled:

.. default-domain:: cpp
.. namespace:: foo
.. class:: something

.. function:: void bar(foo::something)

Environment Information

Platform:              linux; (Linux-6.10.3-arch1-2-x86_64-with-glibc2.40)
Python version:        3.12.4 (main, Jun  7 2024, 06:33:07) [GCC 14.1.1 20240522])
Python implementation: CPython
Sphinx version:        8.0.2
Docutils version:      0.21.2
Jinja2 version:        3.1.4
Pygments version:      2.18.0

Sphinx extensions

No response

Additional context

No response

Start with the minimal index.rst reproduction and empty conf.py, then trace the C++ domain's generated signature nodes and cross-reference resolution. Run the example with nitpicky mode enabled and verify that foo::something links to the class without warning about foo.