Record ignored errors from non-ignored files

Before this change, fine-grained builds could spuriously mark ignored
legitimate errors as "unused ignores".

By keeping track of these ignored errors we ensure that enough analysis
is done to know that the ignored lines are actually useful.

We have to change is_errors_for_file so that we don't consider files as
faulty when all of their errors were hidden.

mypy/errors.py

@@ -503,6 +503,9 @@ def add_error_info(self, info: ErrorInfo) -> None:
                         self.used_ignored_lines[file][scope_line].append(
                             (info.code or codes.MISC).code
                         )
+                        if file not in self.ignored_files:
+                            info.hidden = True
+                            self._add_error_info(file, info)
                         return
             if file in self.ignored_files:
                 return
@@ -804,8 +807,9 @@ def blocker_module(self) -> str | None:
         return None
 
     def is_errors_for_file(self, file: str) -> bool:
-        """Are there any errors for the given file?"""
-        return file in self.error_info_map
+        """Are there any visible errors for the given file?"""
+        errors = self.error_info_map.get(file, [])
+        return any(error.hidden is False for error in errors)
 
     def prefer_simple_messages(self) -> bool:
         """Should we generate simple/fast error messages?