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

https://issues.dlang.org/show_bug.cgi?id=13701
The first instance field assignment is automatically handled as the field initialization. It's consistent with:

```d
struct S {
    immutable(int) v;
    this(int n) {
        v = 20;  // initializing
        v = 30;  // assignment -> cannot modify immutable
    }
}
```