[REG2.061] Issue 13701 - Associative array values ignore immutability

https://issues.dlang.org/show_bug.cgi?id=13701
>Sure, but would that also work for a library AA where this happens through opIndexAssign?

In my library AA if `opIndexAssign` can't assign new value to a pair (for example value is immutable), AA performs _reinitializing_ of this value with the new value. (AA calls dtor for it, if needed, copies new value and calls postblit if needed). We may allow it, because we allow to remove value and insert new value with the same key.
Of course, if AA type marked with const (not only key or value) assigning a new value is not allowed.