Instantiation of nested structs should be allowed outside their parent scope

[Stanislav Blinov]

Just make it `@system`, and even then ONLY if something is 
actually getting instantiated and it's not just a CTFE test. 
Unless I'm missing something, that should be the behavior. The 
'cannot access frame pointer' blah SO needs to go away...

Rationale: instantiating things is super useful for compile time 
tests and inference. Nested structs complicate that to wazoo with 
that failure.

If you're not convinced, I encourage you to try and define e.g.

```d
enum bool isNothrowCopyable(To, From) = /* ??? */
```

without reimplementing copy initialization by hand. It should be 
a trivial test. Just test copy-initialization of a union field. 
But no. Not when nested structs are involved. And they can get 
involved indirectly.