[clang] [clang] Do not substitute parameter pack while retaining the pack expansion (PR #108197)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Other than the test, the idea of the change does seem correct, here's the relevant comment from `Sema.h`:
```cpp
/// The current index into pack expansion arguments that will be
/// used for substitution of parameter packs.
///
/// The pack expansion index will be -1 to indicate that parameter packs
/// should be instantiated as themselves. Otherwise, the index specifies
/// which argument within the parameter pack will be used for substitution.
int ArgumentPackSubstitutionIndex;
```

It looks clear that when `RetainExpansion == true`, we would like the behavior that `ArgumentPackSubstitutionIndex == -1` produces. And given that we already forget the particular partially substituted pack, it's no surprise that using `ArgumentPackSubstitutionIndex` gives us an out-of-bounds access.

Maybe there are certain intricacies I am not getting, but I believe your direction to put this into `ForgetPartiallySubstitutedPackRAII` is the right way forward.

cc @zygoloid @shafik @cor3ntin in case they want to chime in.

https://github.com/llvm/llvm-project/pull/108197