Add regression test for dead code elimination with drop + panic
Add a codegen test for rust-lang/rust#114532.
The bug was that dead code elimination failed when a `Drop` impl contained a `panic!` and a potentially-panicking external function was called after the value was created. This was fixed since 1.82 but no regression test was added.
The test verifies that `foo()` compiles to just a call to `unknown()` + `ret void`, with no panic or panicking call in the function body.
Closesrust-lang/rust#114532