OSX support in dmd appears to be having problems

[Richard (Rikki) Andrew Cattermole]

Forwarding from Iain that got posted on my (unrelated) PR:

Managed to get round to drilling down a bit, issue is shared structs, so 
may not be a regression afterall.

```d
struct AllocatorList
{
     void[] allocators;
     void allocate() { } // b
     // p this
     // $1 = (test.AllocatorList &) @0x7fffffffd4d0: {allocators = 0x0}
}

shared struct SharedAllocatorList
{
     void[] allocators;
     void allocate() { } // b
     // p this
     // $2 = (<unknown type in /build/dlang/test, CU 0x0, DIE 0x178> &) 
@0x7fffffffd4e0:
     // <unknown type in /build/dlang/test, CU 0x0, DIE 0x178>
}

void main()
{
     AllocatorList a;
     a.allocate();
     SharedAllocatorList b;
     b.allocate();
}
```