Handle generics appearing within quoted return annotations

This ensures that these quoted likely forwards references in
return type annotations behave like their non-quoted equivalents.

I suspect there may be other places which will need similar
adjustments, which may mean that we should push the conversion
a layer closer to the parsing (perhaps in `py__annotations__`?).

One case I know that this doesn't solve (but which likely needs
similar adjustment) is generics in return types of comment-style
annotations. They're less likely and may not be worth supporting
since all supported Python versions can use the in-syntax spelling
for annotations at this point.

jedi/inference/gradual/annotation.py

@@ -222,6 +222,18 @@ def infer_return_types(function, arguments):
             match.group(1).strip()
         ).execute_annotation()
 
+    elif annotation.type == 'string':
+        annotation = _get_forward_reference_node(
+            context,
+            context.inference_state.compiled_subprocess.safe_literal_eval(
+                annotation.value,
+            ),
+        )
+        # The forward reference tree has an additional root node ('eval_input')
+        # that we don't want. Extract the node we do want, that is equivalent to
+        # the nodes returned by `py__annotations__` for a non-quoted annotation.
+        annotation = annotation.children[0]
+
     unknown_type_vars = find_unknown_type_vars(context, annotation)
     annotation_values = infer_annotation(context, annotation)
     if not unknown_type_vars:

test/completion/pep0484_generic_passthroughs.py

@@ -58,6 +58,12 @@ def typed_bound_generic_passthrough(x: TList) -> TList:
 
     return x
 
+# Forward references are more likely with custom types, however this aims to
+# test just the handling of the quoted type rather than any other part of the
+# machinery.
+def typed_quoted_generic_passthrough(x: T) -> 'List[T]':
+    return [x]
+
 
 for a in untyped_passthrough(untyped_list_str):
     #? str()
@@ -146,6 +152,16 @@ for q in typed_bound_generic_passthrough(typed_list_str):
     q
 
 
+for r in typed_quoted_generic_passthrough("something"):
+    #? str()
+    r
+
+for s in typed_quoted_generic_passthrough(42):
+    #? int()
+    s
+
+
+
 class CustomList(List):
     def get_first(self):
         return self[0]