[Issue 24211] Const nested function can mutate context

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24211

--- Comment #2 from Basile-z <b2.temp at gmx.com> ---
A bit more of analysis:

function attributes apply to the context. The fact that this works is more
related to another problem that is "function signatures are wrong"

```d
struct S
{
    function f() const {}
}
```

is actually more like

```d
struct S {}

function f(const S* s){}
```

The storage class cannot be put on a parameter that dont exist, so the D
solution is to
make the hidden `this` const using trailing attributes.

That should apply to nested function too, those that captures `this`.

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