Special case to preserve/ban `implicit-optional` when baselined

Turning off implicit-optional and baselining will introduce a bunch of false errors
```py
def foo(value: str = None): # baselined "assignment" error
bar: str = value # no error because type of value is `str`
```
We should implement a special case to either:
- straight up ban this code even when baselining
- preserve the implicit optional

Start by reproducing the shown `implicit-optional` example with baselining enabled, then trace the checker paths that handle implicit optional types and baselined errors. Done means the project has a decided, tested behavior: either this pattern remains banned during baselining or the implicit optional is preserved without producing the false downstream error.

Written by the indexing model from the issue text.