[SimplifyCFG] When eliminating `unreachable` landing pads, mark `call`s as `nounwind`

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.