C++ 20 support

[s9105947]

Description

Compiling with C++20 issues (unexpected) warnings.

Steps to reproduce

Include scorep_plugin_cxx_wrapper header in a C++20 project

Expected Behavior

successful compilation, no warnings issued

Actual Behavior

compilation is successful,
but these warnings are issued (by g++):

In file included from scorep_plugin_topdown/lib/scorep_plugin_cxx_wrapper/include/scorep/plugin/plugin.hpp:33,
                 from scorep_plugin_topdown/include/plugin.hpp:9,
                 from scorep_plugin_topdown/src/plugin.cpp:1:
scorep_plugin_topdown/lib/scorep_plugin_cxx_wrapper/include/scorep/plugin/base.hpp: In static member function ‘static SCOREP_Metric_Plugin_Info scorep::plugin::base<Child, Args>::get_info()’:
scorep_plugin_topdown/lib/scorep_plugin_cxx_wrapper/include/scorep/plugin/base.hpp:342:13: warning: ‘++’ expression of ‘volatile’-qualified type is deprecated [-Wvolatile]
  342 |             tls_dummy++;
      |             ^~~~~~~~~
scorep_plugin_topdown/lib/scorep_plugin_cxx_wrapper/include/scorep/plugin/base.hpp: In instantiation of ‘static SCOREP_Metric_Plugin_Info scorep::plugin::base<Child, Args>::get_info() [with Child
= topdown_plugin; Args = {scorep::plugin::policy::sync, scorep::plugin::policy::per_thread, scorep::plugin::policy::scorep_clock, tmam_metric_t_policy}; SCOREP_Metric_Plugin_Info = SCOREP_Metric_Plugin_Info]’:
scorep_plugin_topdown/src/plugin.cpp:3:1:   required from here
scorep_plugin_topdown/lib/scorep_plugin_cxx_wrapper/include/scorep/plugin/base.hpp:342:13: warning: ‘++’ expression of ‘volatile’-qualified type is deprecated [-Wvolatile]

Notes

Can be circumvented by silencing these warnings:

#pragma GCC diagnostic push 
#pragma GCC diagnostic ignored "-Wvolatile"
#include <scorep/plugin/plugin.hpp>
#pragma GCC diagnostic pop