CommandLine: add and use cl::SubCommand::get{All,TopLevel}

Prefer using these accessors to access the special sub-commands
corresponding to the top-level (no subcommand) and all sub-commands.

This is a preparatory step towards removing the use of ManagedStatic:
with a subsequent change, these global instances will be moved to
be regular function-scope statics.

It is split up to give downstream projects a (albeit short) window in
which they can switch to using the accessors in a forward-compatible
way.

Differential Revision: https://reviews.llvm.org/D129118