C++, crash on certain duplicate declarations

[jakobandersen]

Describe the bug
When a function is added after another entity with the same name is added they can erroneously co-exist. When adding a third non-function entity with the same name, Sphinx crashes:

Exception occurred:
  File "/home/jla/repos/sphinx/sphinx/domains/cpp.py", line 4354, in _add_symbols
    assert len(withDecl) <= 1
AssertionError

To Reproduce
Steps to reproduce the behavior:

.. cpp:struct:: A
.. cpp:function:: void A()
.. cpp:struct:: A

Expected behavior
A warning about duplicate declaration should be given for line 2 and 3.

[jakobandersen]

Fixes in the 3.2.x branch.