Fix pass driver's dump_pass_list_ and print_pass_list_.

The lists were allocated with new char[], so they should
have been held by std::unique_ptr<const char[]> rather than
std::unique_ptr<const char>. However, it's much cleaner with
std::string.

Change-Id: Ie7c604773272194345f5e6e3c4803c3a914edf99