Fixed an issue where you couldn't delete a user defined regex, python, or...

Fixed an issue where you couldn't delete a user defined regex, python, or multi-word command by adding a new "command delete" command.

This new command will delete user defined regular commands, but not aliases. We still have "command unalias" to remove aliases as they are currently in different buckets. Appropriate error messages are displayed to inform the user when "command unalias" is used on removable user defined commands that points users to the "command delete" command.

Added a test to verify we can remove user defined commands and also verify that "command unalias" fails when used on a user defined command.
<rdar://problem/18248300>

llvm-svn: 225535