Summary
--
This PR fixes the issue pointed out by @JelleZijlstra in
https://github.com/astral-sh/ruff/pull/17101#issuecomment-2777480204.
Namely, I conflated two very different errors from CPython:
```pycon
>>> def m[T](x: (yield from 1)): ...
File "<python-input-310>", line 1
def m[T](x: (yield from 1)): ...
^^^^^^^^^^^^
SyntaxError: yield expression cannot be used within the definition of a generic
>>> def m(x: (yield from 1)): ...
File "<python-input-311>", line 1
def m(x: (yield from 1)): ...
^^^^^^^^^^^^
SyntaxError: 'yield from' outside function
>>> def outer():
... def m(x: (yield from 1)): ...
...
>>>
```
I thought the second error was the same as the first, but `yield` (and
`yield from`) is actually valid in this position when inside a function
scope. The same is true for base classes, as pointed out in the original
comment.
We don't currently raise an error for `yield` outside of a function, but
that should be handled separately.
On the upside, this had the benefit of removing the
`InvalidExpressionPosition::BaseClass` variant and the
`allow_named_expr` field from the visitor because they were both no
longer used.
Test Plan
--
Updated inline tests.