Add a failing test for type ignores in dmypy

This catches a regression caused by the previous commits where "type:
ignore" comments are erroneously marked as unused in re-runs of dmypy.

As far as I can tell, this only happens in modules which contain an
import that we don't know how to type (such as a module which does not
exist), and a submodule which is unused.

test-data/unit/daemon.test

@@ -611,3 +611,24 @@ from foo.empty import *
 if False:
     a = 1
 a
+
+[case testReturnTypeIgnoreAfterUnknownImport]
+-- Return type ignores after unknown imports and unused modules are respected on the second pass.
+$ dmypy start -- --warn-unused-ignores --no-error-summary
+Daemon started
+$ dmypy check -- foo.py
+foo.py:2: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import]
+foo.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+$ dmypy check -- foo.py
+foo.py:2: error: Cannot find implementation or library stub for module named "a_module_which_does_not_exist"  [import]
+foo.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
+== Return code: 1
+
+[file unused/__init__.py]
+[file unused/empty.py]
+[file foo.py]
+from unused.empty import *
+import a_module_which_does_not_exist
+def is_foo() -> str:
+    return True  # type: ignore