[Issue 17972] __traits(getLinkage) doesn't work for classes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17972

--- Comment #2 from Jacob Carlborg <doob at me.com> ---
What about using the existing fields in ClassDeclaration: "cpp", "objc" and so
on [1]? I know it's not exactly the same.

[1] https://github.com/dlang/dmd/blob/master/src/ddmd/dclass.d#L215-L218

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