[lldb] Add OS plugin property for reporting all threads (#123145)

Currently, an LLDB target option controls whether plugins report all
threads. However, it seems natural for this knowledge could come from
the plugin itself. To support this, this commits adds a virtual method
to the plugin base class, making the Python OS query the target option
to preserve existing behavior.

lldb/source/Target/Process.cpp

@@ -1182,7 +1182,7 @@ void Process::UpdateThreadListIfNeeded() {
           // See if the OS plugin reports all threads.  If it does, then
           // it is safe to clear unseen thread's plans here.  Otherwise we
           // should preserve them in case they show up again:
-          clear_unused_threads = GetOSPluginReportsAllThreads();
+          clear_unused_threads = os->DoesPluginReportAllThreads();
 
           // Turn off dynamic types to ensure we don't run any expressions.
           // Objective-C can run an expression to determine if a SBValue is a