[lldb] Return StringRef from PluginInterface::GetPluginName

There is no reason why this function should be returning a ConstString.

While modifying these files, I also fixed several instances where
GetPluginName and GetPluginNameStatic were returning different strings.

I am not changing the return type of GetPluginNameStatic in this patch, as that
would necessitate additional changes, and this patch is big enough as it is.

Differential Revision: https://reviews.llvm.org/D111877

lldb/source/Plugins/OperatingSystem/Python/OperatingSystemPython.cpp

@@ -140,11 +140,6 @@ DynamicRegisterInfo *OperatingSystemPython::GetDynamicRegisterInfo() {
   return m_register_info_up.get();
 }
 
-// PluginInterface protocol
-ConstString OperatingSystemPython::GetPluginName() {
-  return GetPluginNameStatic();
-}
-
 bool OperatingSystemPython::UpdateThreadList(ThreadList &old_thread_list,
                                              ThreadList &core_thread_list,
                                              ThreadList &new_thread_list) {