[lldb] Fix use-after-free in jThreadsInfo stop info cache (#223488)
WillPublicStop iterates over the array borrowed from m_jthreadsinfo_sp
while WillResume concurrently resets the pointer on another thread,
freeing the array during the walk.
Protect the cached stop info with a mutex and copy the shared pointer
out before processing. Also modernize m_shared_cache_info to Guarded.
rdar://186776438