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

Summary: Now -exec-next command uses SB API for stepping over.

Reviewers: aprantl, clayborg, stella.stamenova, labath

Reviewed By: aprantl, clayborg, labath

Subscribers: labath, ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D47797

llvm-svn: 334215