Clean-up classes nested in functions (#21478)
Fixes https://github.com/python/mypy/issues/6422
Fixes https://github.com/python/mypy/issues/13024
TBH the current situation is embarrassing. I decided to _finally_ clean
this all up and unify various cases. Now we have same simple rules used
by all classes, both regular (first three) and magic:
* Use the name mangling for everything inside a top-level function.
* Do not mangle `defn.name` only `defn.fullname`.
* Always store classes nested in functions in global symbol table (using
enclosing class was only adding unnecessary complications)
* In cases of name mismatch (like `One = NamedTuple("Other", ...)` etc)
always use the `var_name` for all purposes.
* In cases of inline base classes use different mangling mechanism
(since those may be nested in functions as well).
