[lldb, lldb-mi] Re-implement MI -exec-continue command.

Summary: Now -exec-continue command uses SB API to resume target's process.

Reviewers: aprantl, clayborg, labath

Reviewed By: clayborg

Subscribers: apolyakov, labath, ki.stfu, llvm-commits, lldb-commits

Differential Revision: https://reviews.llvm.org/D47415

llvm-svn: 334350