[Issue 21974] importC: Error: undefined identifier '__builtin_va_list'

[d-bugmail at puremagic.com]

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

--- Comment #9 from Walter Bright <bugzilla at digitalmars.com> ---
These also need to be added per https://issues.dlang.org/show_bug.cgi?id=22307
:

__builtin_va_start
__builtin_va_arg
__builtin_va_end

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