[lldb] Lower GetRealStopInfo into ThreadPlanCallFunction (NFC)

`GetRealStopInfo` has only one call site, and in that call site a reference to the
concrete thread plan is available (`ThreadPlanCallUserExpression`), from which
`GetRealStopInfo` can be called.

Differential Revision: https://reviews.llvm.org/D96687

GitOrigin-RevId: 2a2464eb7651dc89290e9e3bf6928c99453bed99
3 files changed