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

https://issues.dlang.org/show_bug.cgi?id=13701
> Your actually calling opIndexAssign so it's not initialization.

No, if `k` is not yet stored in `aa`, `aa[k] = v` will be an initialization.
See: https://issues.dlang.org/show_bug.cgi?id=6178

Today, dmd does not allow *incremental initializing* anyway. If you *partially* initialize an instance field, the other part of the field will be left as T.init. By the same logic:

```d
struct Point { int x, y; }
struct S {
    immutable int[2] sa;
    immutable Point pt;
    this(int) {
        sa[0] = 1;  // initializing, OK
        sa[1] = 2;  // assignment, NG
        pt.x = 1;   // initializing, OK
        pt.y = 2;   // assignment, NG
    }
}
```

This is necessary limitation not to break type system.