[Dlang-internal] DIP1000 discussion and testing

[Mathias Lang via Dlang-internal]

```
T* foo (T) (T* arg)
{
     return arg;
}

int* escape () @safe
{
     int b;
     return foo(&b);
}

void main () @safe
{
     int* ptr = escape();
}
```

Compiles with `./src/dmd -transition=safe -run test.d` @ 
ac6f655030f814cd352a33b2c9490df16c84459d