[lldb] Tablegenify thread commands and fix completion bug for thread step-*

Beside turning the options into the new tablegen format, this patch
also fixes that a few commands had source file completions for the
"count" and "end-linenumber" arguments (which both accepted only
integers). Reason for that are that somehow we added a '1' instead
of our usual '0' value to the initial value for completion.

llvm-svn: 366425