[Issue 19278] extern(C++, "name") doesn't accept expressions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19278

--- Comment #1 from Manu <turkeyman at gmail.com> ---
String expressions don't work (obviously)
```
extern (C++, "std" ~ (UseCxx11Namespace ? "::__1" : "")):
```

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