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

[d-bugmail at puremagic.com]

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

--- Comment #4 from Walter Bright <bugzilla at digitalmars.com> ---
As you say, it's problematic to add the __builtin_va_list to cparse, as cparse
does not have access to the types.

A simple approach is to have the semantic routines start by adding the
declaration for it, rather than cparse.

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