Expand semantic syntax errors for invalid walruses (#25415)
## Summary
Python rejects assignment expressions in several comprehension contexts,
but we previously only reported the case in which a walrus rebound a
comprehension variable.
As a result, we accepted invalid walruses in the iterable and class-body
portions clause of a comprehension, and missed some rebinding cases in
filter clauses and nested comprehensions. It turns out the rules are
really complicated!
For example, we now report the two newly modeled syntax errors:
```py
# error: [invalid-syntax] "assignment expression cannot be used in a comprehension iterable expression"
[x for x in (values := [1])]
class C:
# error: [invalid-syntax] "assignment expression within a comprehension cannot be used in a class body"
[(value := item) for item in [1]]
```
We also catch previously missed instances of the existing rebinding
error:
```py
# error: [invalid-syntax] "assignment expression cannot rebind comprehension variable"
[x for x in [1] if (x := 0)]
```
In general, I decided not to worry about emitting multiple diagnostics
for a single error. These are rare, they're fatal, and it added a lot of
complexity (so in some cases we may emit two errors for a walrus that is
invalid for two reasons).
