[clang] [Clang][Cygwin] Use correct BuiltinVaListKind. (PR #143166)

[via cfe-commits]

jeremyd2019 wrote:

I don't know - it's easy enough to do *in vivo*
```c
#include <stdio.h>
#include <stdarg.h>

void test(const char * fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        vprintf(fmt, args);
        va_end(args);
}

int main(void)
{
        test("asdf %d %s\n", 1, 1==1 ? "TEST" : "TESTS");
        return 0;
}
```

Before this change
```
asdf -13288 (null)
```

After
```
asdf 1 TEST
```

https://github.com/llvm/llvm-project/pull/143166