[Issue 21946] importC: Support parsing __extension__ keyword

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21946

Iain Buclaw <ibuclaw at gdcproject.org> changed:

           What    |Removed                     |Added
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           See Also|                            |https://issues.dlang.org/sh
                   |                            |ow_bug.cgi?id=22598

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