Add translation rules for variadic libc calls

[lucic71]

Variadic rules are defined with the following syntax:

template <typename... Args>
int f1(int a0, unsigned long a1, Args... args) {
  return ioctl(a0, a1, args...);
}

and

unsafe extern "C" {
    fn f1(a0: i32, a1: u64, ...) -> i32;
}

For the C++ side, use a template argument pack. This makes it clear that the function call is variadic. For the Rust side, use a variadic function inside an extern block, that's the only place Rust allows variadic functions.

I added a new attribute in the IR, called is_extern. It's used by the new Mapper::IsLibcPassthrough. A libc passthrough is a rule that it's declared in Rust extern block. In the above example ioctl is defined as a passthrough for libc::ioctl. In the future, we can use this for non-variadic rules as well.

[lucic71]

@joaotgouveia please take a look at the changes in cpp2rust/cpp_rule_preprocessor.cpp

[lucic71]

@nunoplopes this is ready for review

[nunoplopes]

Uhm, this seems like a hack that works for a few unsafe rules, but it's of no use for refcount. Doesn't feel like the solution we need.

[lucic71]

Uhm, this seems like a hack that works for a few unsafe rules, but it's of no use for refcount. Doesn't feel like the solution we need.

It works for all variadic unsafe rules. I designed this only with unsafe in mind. Probably for refcount we will need another solution.

Let me think if I can come up with something that would also work for refcount.

[joaotgouveia]

I think that this can be replaced by clang::TemplateArgument::getEmptyPack()