Add the ability to provide a "count" option to the various "thread step-*" operations.  Only
step-inst and step-inst are currently supported, the rest just warn that they are not supported
if you try to provide a count.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212559 91177308-0d34-0410-b5e6-96231b3b80d8