CheckerGccPlugins: Various updates.
check_thread_safety_p.cxx:
Need to ensure that a decl callback really gets a decl --- with the using ns hack
applied, it could possibly get a USING_STMT. Fixes crash observed with gcc8.
thread_plugin.cxx: Add another exception to thread-safety checking needed for root 6.12.
gaudi_inheritance_plugin.cxx: Add ConditionsCleanerTest to gaudi checker whitelist. Don't warn about Gaudi components in AthenaKernel.