[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