Always disallow assignments to constants (#57567)
We have historically allowed the following without warning or error:
```
const x = 1
x = 1
```
As well as
```
const x = 1
x = 2
```
with a UB warning.
In 1.12, we made the second case error, as part of the general binding
partition changes, but did not touch the first case, because it did not
have the warning.
I think this made a reasonable amount of sense, because we essentially
treated constants as special kinds of mutable globals (to which
assignment happened to be UB).
However, in the 1.12+ design, constants and globals are quite sepearate
beasts, and I think it makes sense to extend the error to the egal case
also, even if it is technically more breaking.
In fact, I already thought that's what I did when I implemented the new
effect model for global assignment, causing #57566. I can't think of a
legitimate reason to keep this special case. For those who want to do
binding replacement, the `const` keyword is mandatory, so the assignment
is now literally always a semantic no-op or an error.
Fixes #57566.
