[LifetimeSafety] False-positive due to assignments involving std::tie

https://godbolt.org/z/q6ToGh779
```cpp
#include
#include

int& reproduce(int Key) {
std::map Cache;
std::map::iterator It = Cache.begin();
bool New;
std::tie(It, New) = Cache.try_emplace(Key);

return It->second;
}
```

`std::tie` on LHS should KILL the iterator `It` value and therefore it shouldn't be considered alive at the point of invalidation.

Start with the Godbolt reproducer in the issue and inspect the LifetimeSafety analysis handling of assignments to std::tie. Confirm how the left-hand-side assignment affects the iterator's lifetime state, then add regression coverage showing that this example no longer produces a false positive.

Written by the indexing model from the issue text.