C domain: References generated by types in function signatures don't use intersphinx

[eric-wieser]

Describe the bug
Over in numpy/numpy#16370, we're getting warnings about missing references to C identifiers even though we know they exist in projects in our intersphinx

To Reproduce
Create an RST file with

.. c:function:: PyObject* MyFunction(PyObject* op)
   Some description

put the following in conf.py

extensions = ['sphinx.ext.intersphinx']
intersphinx_mapping = {
    'python': ('https://docs.python.org/dev', None),
}

Build the sphinx docs with reference warnings enabled. The output log will contain

c:identifier reference target not found: PyObject

which is strange, because the https://docs.python.org/dev/objects.inv definitely contains an entry for PyObject.

Expected behavior
No warning

Your project
Link to your sphinx project, or attach zipped small project sample.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment info

• Python version: 3.8
• Sphinx version: 3.2.1
• Sphinx extensions: sphinx.ext.intersphinx

[jakobandersen]

Fixed in 3.x (to become v3.5). The fix is only on the client side so you can simply recompile your own project with the new version.