[Clang] SIGSEGV ICE with lambda init-capture pack

https://godbolt.org/z/j5nbG4Mdd
```cxx
template struct V {};
void init_capture_pack() {
auto L = [](auto... z) {
return [... w = z, y...](struct x1)
{};
};
L(V<0>{});
}
```
Seems to be introduced since clang 17.1.0

Start with the linked Compiler Explorer reproduction and run the C++ snippet across Clang versions, particularly around 17.1.0, to confirm when the SIGSEGV ICE appears. Done means the reproducer no longer crashes Clang and a regression test covers the lambda init-capture pack case.

Written by the indexing model from the issue text.