Struct field destructor not called when exception is thrown in the main struct destructor

[tchaloupka]

Found a pretty nasty bug in vibe-d: 
https://github.com/vibe-d/vibe.d/issues/2484

And it's caused by this behavior.

```D
import std;

struct Foo {
     Bar bar;
     bool err;

     ~this() {
         // scope(failure) destroy(bar); // < this fixes the Bar 
destructor call
         enforce(!err, "Test err");
     }
}

struct Bar {
     static int refs;
     ~this() { refs--; }
}

void main()
{
     {
         Foo f;
         Bar.refs = 1;
     }
     assert(Bar.refs == 0);

     try () {
         Foo f;
         f.err = true;
         Bar.refs = 1;
     }();
     catch (Exception ex) {}
     assert(Bar.refs == 0);
}
```

So when the exception is thrown within Foo destructor (and it's 
bad on it's own but can easily happen as destructors aren't 
nothrow @nogc by default).

Is this behavior expected?