[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 #13 from Walter Bright <bugzilla at digitalmars.com> ---
https://github.com/dlang/dmd/pull/13107 solves the specific __builtin_va_list
problem, but not the general problem.

The general problem could be addressed by importing core.stdc.stdarg into the C
semantic routines and thereby hijacking the D implementation for use with the C
code.

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