ext.autodoc: fix formatting instance annotations

Currently `format_annotation` fails on instances, because instances don't have
`__module__` and `__qualname__` attributes.  Defer building the
`qualified_name` of an annotation until we have established that the annotation
is a type.

tests/typing_test_data.py

@@ -46,3 +46,11 @@ def f7(x: Callable[[int, str], int]) -> None:
 
 def f8(x: Tuple[int, str], y: Tuple[int, ...]) -> None:
     pass
+
+
+class CustomAnnotation:
+    def __repr__(self):
+        return 'CustomAnnotation'
+
+def f9(x: CustomAnnotation(), y: 123) -> None:
+    pass