Fix anonymous struct/union usage in C domain

Sphinx's C domain parser expects '.' to be used to separate nested
names. This is currently broken when there are anonymous structs/unions
in the source, where the Doxygen XML provides some nested name with
"::@" in it, which  fails to be parsed properly. This commit adds a
check and does the proper substitution before sending data back to the
domain parser.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

breathe/renderer/sphinxrenderer.py

@@ -1707,9 +1707,12 @@ def visit_variable(self, node) -> List[Node]:
             if len(initializer) != 0:
                 options['value'] = initializer
         else:
+            typename = ''.join(n.astext() for n in self.render(node.get_type()))
+            if dom == 'c' and '::' in typename:
+                typename = typename.replace('::', '.')
             declaration = ' '.join([
                 self.create_template_prefix(node),
-                ''.join(n.astext() for n in self.render(node.get_type())),  # type: ignore
+                typename,
                 name,
                 node.get_argsstring(),
                 self.make_initializer(node)