[Issue 18625] New: mixin(__MODULE__) in string mixin results in undefined identifier in certain cases

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18625

          Issue ID: 18625
           Summary: mixin(__MODULE__) in string mixin results in undefined
                    identifier in certain cases
           Product: D
           Version: D2
          Hardware: x86
                OS: Windows
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

This works: 

```
module asdf;

mixin(`static assert(mixin(__MODULE__).stringof == "module "~__MODULE__);`);
```

This does not work:

```
module y.asdf;

mixin(`static assert(mixin(__MODULE__).stringof == "module "~__MODULE__);`);
```

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