[Issue 18544] Thread-safety of "proto" GC before initialisation

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18544

--- Comment #3 from rm <rymrg at memail.com> ---
Example program that makes use of addRoots before anything is allocated.
The D following lib is called from multiple threads in C++.

```D
import std;
extern(C) {
   void cFunc( ){
      Foo foo;
      foo.inc;
   }
}

struct Foo{
   Array!int impl;
   void inc(){
       impl.insertBefore(impl[], 0);
   }
}
```

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