Calling `__abnormal_termination` terminates clang abnormally

the following code crashes

```cpp
void a() { _abnormal_termination(); }
```

https://compiler-explorer.com/z/5YTdre7z3

The issue is that `_abnormal_termination` is only designed to be called from a `finally` block but this requirement is not enforced

Bug initially found by @ojhunt

Start with the minimal C++ reproducer calling _abnormal_termination() outside a finally block and compare the behavior shown in the linked Compiler Explorer example. Trace how clang handles this entry point and identify where the requirement for a finally block is or is not checked. Done means the reproducer no longer crashes clang abnormally.

Written by the indexing model from the issue text.