ICE when explicit object parameter be a function parameter pack

[hewillk]

struct S {
  auto f(this auto self...) {  }
};

int main() {
  S{}.f(0);
}

https://godbolt.org/z/bP8f6hzGs

Clang will crash with the above code. This should be a legal variadic function IMO.

[Sirraide]

Looks like we have an out-of-bounds access in Sema::CheckNonDependentConversions.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hewill Kang (hewillk)

```cpp struct S { auto f(this auto self...) { } };

int main() {
S{}.f(0);
}

https://godbolt.org/z/bP8f6hzGs

Clang will crash with the above code. This should be a legal variadic function IMO.
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