[lldb-mi] Re-implement a few MI commands.

Summary: This patch updates exec-next-instruction, exec-step-instruction,
exec-finish, exec-interrupt commands to use SB API instead of HandleCommand.

Reviewers: aprantl, clayborg

Reviewed By: aprantl

Subscribers: ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D48520

llvm-svn: 336155