[syntax-errors] Allow yield in base classes and annotations (#17206)

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.

crates/ruff_python_parser/resources/inline/err/invalid_annotation_class.py

@@ -1,6 +1,4 @@
 class F[T](y := list): ...
-class G((yield 1)): ...
-class H((yield from 1)): ...
 class I[T]((yield 1)): ...
 class J[T]((yield from 1)): ...
 class K[T: (yield 1)]: ...      # yield in TypeVar

crates/ruff_python_parser/resources/inline/err/invalid_annotation_function.py

@@ -1,13 +1,9 @@
 def f[T]() -> (y := 3): ...
 def g[T](arg: (x := 1)): ...
 def h[T](x: (yield 1)): ...
-def i(x: (yield 1)): ...
 def j[T]() -> (yield 1): ...
-def k() -> (yield 1): ...
 def l[T](x: (yield from 1)): ...
-def m(x: (yield from 1)): ...
 def n[T]() -> (yield from 1): ...
-def o() -> (yield from 1): ...
 def p[T: (yield 1)](): ...      # yield in TypeVar bound
 def q[T = (yield 1)](): ...     # yield in TypeVar default
 def r[*Ts = (yield 1)](): ...   # yield in TypeVarTuple default

crates/ruff_python_parser/resources/inline/err/invalid_annotation_function_py314.py

@@ -0,0 +1,8 @@
+# parse_options: {"target-version": "3.14"}
+def f() -> (y := 3): ...
+def g(arg: (x := 1)): ...
+def outer():
+    def i(x: (yield 1)): ...
+    def k() -> (yield 1): ...
+    def m(x: (yield from 1)): ...
+    def o() -> (yield from 1): ...