[Issue 20805] C++ mangling mismatch with templates and namespaces

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20805

--- Comment #2 from Andrej Mitrovic <andrej.mitrovich at gmail.com> ---
Simplified test-case:

```
extern (C++, `xdr`)
struct xvector(T)
{
}

extern (C++)
public void push_back1 (ref xvector!(xvector!ubyte), ref xvector!ubyte);

extern (C++)
public void push_back2 ()(ref xvector!(xvector!ubyte), ref xvector!ubyte);

void main ()
{
    xvector!(xvector!ubyte) vec;
    xvector!ubyte val;

    vec.push_back1(val);
    vec.push_back2(val);
}
```

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