[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

--- Comment #14 from Walter Bright <bugzilla at digitalmars.com> ---
https://github.com/dlang/dmd/pull/13205

adds support for __builtin_va_start and __builtin_va_end

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