[Issue 21944] importC: Support parsing # line marker directive extensions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21944

--- Comment #5 from Walter Bright <bugzilla at digitalmars.com> ---
The Digital Mars preprocessor, as well as the Warp preprocessor, emit
linemarker directives.

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