[Issue 21974] ImportC: support __builtin_va_list, __builtin_va_start, __builtin_va_arg, __builtin_va_end

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21974

Walter Bright <bugzilla at digitalmars.com> changed:

           What    |Removed                     |Added
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           See Also|                            |https://issues.dlang.org/sh
                   |                            |ow_bug.cgi?id=22589

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