[Issue 24509] importC cannot handle _stdcall Function Calling Convention with single heading underscore

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24509

--- Comment #2 from Ferhat Kurtulmuş <aferust at gmail.com> ---
My fix:
https://github.com/aferust/dmd/commit/23e78aedb75931a27089ea1912d5fd56dd2d0c50

This is important because the MSVC compiler accepts both _stdcall and
__stdcall, one or two underscores.

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