Added a new Option template, cl::debug_opt, to the
CommandLine parser that aliases to cl::opt in debug builds, but
decays to a simple class containing only the initialized DataType in
debug_opts are also removed from the parser in release builds, so
they don't take up any additional memory, and are designed to be a
drop-in replacement for cl::opt.
cl::OptionCategory and cl::SubCommand also have debug versions
which disappear in release builds.