[lldb-mi] Re-implement MI -exec-step command.

Summary: Now -exec-step uses SB API instead of HandleCommand hack.

Reviewers: aprantl, clayborg, labath, stella.stamenova

Reviewed By: aprantl

Subscribers: ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D47838

llvm-svn: 334364