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

[d-bugmail at puremagic.com]

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

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Patch sent that recommends users to use `-P` to turn off the generation of
linemarkers in the output from the preprocessor.  This saves us having to
support the extension.

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