[llvm] [WebAssembly] Fix rethrow's index calculation (PR #114693)

[Reid Kleckner via llvm-commits]

https://github.com/rnk approved this pull request.

The selection DAG builder changes look good to me, it's pretty straightforward to pass through the information we're already modeling.

Since C++11, destructors are implicitly `noexcept(true)`, so this has become a pretty obscure edge case, since if I understand the scenario right, an uhandled exception has to unwind out of an inlined destructor call.

https://github.com/llvm/llvm-project/pull/114693