[llgo] cmd/llgoi: use line editor

Summary:
This diff adds line-editing to llgoi, by
vendoring and using github.com/peterh/liner.
I have only implemented the basics here;
follow-ups will come later to add persisted
history, and completion,

Reviewers: pcc

Reviewed By: pcc

Subscribers: axw, llvm-commits

Differential Revision: http://reviews.llvm.org/D9811

llvm-svn: 238100