[red-knot] Invalid assignments to attributes (#15613)
## Summary
Raise "invalid-assignment" diagnostics for incorrect assignments to
attributes, for example:
```py
class C:
var: str = "a"
C.var = 1 # error: "Object of type `Literal[1]` is not assignable to `str`"
```
closes #15456
## Test Plan
- Updated test assertions
- New test for assignments to module-attributes
