CheckerGccPlugins: Various updates to the thread-safety static checker to fix false positives.

Should fix warnings seen in recent master builds.

• Ok to use static mutex objects.
• When we warn about an expression, include the type in the warning.
• Recognize boost:shared_mutex and std::shared_timed_mutex as mutex types. Recognize boost::thread_specific_ptr.
• Fix issues with marking the LHS of an assignment as thread-safe.
• More const-checker fixes for pointers to pointers.
• Don't warn about discarding const on a call to delete, or a virtual dtor.