[CommandObject] Use GetDebugger() helper method (NFC)

In r359354 a GetDebugger() method was added to the CommandObject class,
so that we didn't have to go through the command interpreter to obtain
the script interpreter. This patch simplifies other call sites where
m_interpreter.GetDebugger() was used, and replaces them with a shorter
call to the new method.

llvm-svn: 359373
diff --git a/lldb/source/Commands/CommandObjectBreakpoint.cpp b/lldb/source/Commands/CommandObjectBreakpoint.cpp
index 82b3fc3..d4ec326 100644
--- a/lldb/source/Commands/CommandObjectBreakpoint.cpp
+++ b/lldb/source/Commands/CommandObjectBreakpoint.cpp
@@ -897,7 +897,7 @@
       const bool show_locations = false;
       bp_sp->GetDescription(&output_stream, lldb::eDescriptionLevelInitial,
                          show_locations);
-      if (target == m_interpreter.GetDebugger().GetDummyTarget())
+      if (target == GetDebugger().GetDummyTarget())
         output_stream.Printf("Breakpoint set in dummy target, will get copied "
                              "into future targets.\n");
       else {