[lldb/API] NFC: Reformat and simplify SBThread::GetStopDescription()

Summary:
This gets rid of some nesting and of the raw char* variable that caused
the memory management bug we hit recently.

This commit also removes the fallback code which should trigger when
the StopInfo provides no stop description. All currently implemented
StopInfos have a `GetDescription()` method that shouldn't return an
empty description.

Reviewers: JDevlieghere, labath, mib

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74157