Let chains tests
Filing this as this marks off two of the open issues in #132833:
* extending the tests for `move-guard-if-let-chain.rs` and `conflicting_bindings.rs` to have chains with multiple let's (one implementation could for example search for the first `let` and then terminate).
* An instance where a temporary lives shorter than with nested ifs, breaking compilation: #103476. This was fixed in the end by the if let rescoping work.
Closes#103476
ccc3f862d6