Lots of progress on the CommandAlias refactoring

This cleans things up such CommandAlias essentially can work as its own object; the aliases still live in a separate map, but are now just full-fledged CommandObjectSPs
This patch also cleans up help generation for aliases, allows aliases to vend their own help, and adds a tweak such that "dash-dash aliases", such as po, don't show the list of options for their underlying command, since those can't be provided anyway

I plan to fix up a few more things here, and then add a test case and proclaim victory

llvm-svn: 263499