[SimplifyCFG] Reapply: when eliminating `unreachable` landing pads, mark `call`s as `nounwind`

This time the change is in it's least intrusive form since only the return
type in prototype for `removeUnwindEdge()` is changed, since only a single
specific caller need that knowledge.

We really can't recover that knowledge, and `nounwind` knowledge,
(and not just a lack of the unwind edge, aka `call` instead of `invoke`),
is e.g. part of the reasoning in e.g. `mayHaveSideEffects()`.

Note that this is call-site-specific knowledge,
just because some callsite had an `unreachable`
unwind edge, does not mean that all will.