[lldb] Minor refinements to ThreadPlan::RestoreThreadState (NFC)

Correct `RestoreThreadState` to a `void` return type. Also, update the signature of its
callee, `Thread::RestoreThreadStateFromCheckpoint`, by updating it to a `void` return
type, and making it non-`virtual`.

Differential Revision: https://reviews.llvm.org/D96688

GitOrigin-RevId: 65d91b40ae1d273586a8265d7cf598f0e56e17a3
5 files changed