[mypyc] Fix crash on multiple nested decorated functions with same name (#20666)
This happens when a nested function has different definitions in
different condition branches. mypyc currently crashes in this case when
calling `get_func_target` in `transform_decorator`
https://github.com/python/mypy/blob/master/mypyc/irbuild/function.py#L118.
The `FuncDef` objects for the definitions after the first one will have
the `original_def` attribute set. This makes `get_func_target` lookup
this original def but it doesn't find it when the original def is a
decorator because registers are only created for `FuncDef`s.
Extracting a `FuncDef` from the original decorator and looking it up
instead fixes the crash.