[PATCH] Introduce line editor library.

[Peter Collingbourne]

  I did consider linenoise. My thoughts are that if we were to implement our own line editor, linenoise would be a good starting point. Unfortunately, it does not implement the style of tab completion (show all completions, rather than cycle through each completion) that I think clients such as clang-query would really benefit from, mainly by being able to teach users the matcher syntax as they use it by showing type information for each completing matcher. While libedit does not natively support tab completion, it gives you just enough control to implement it in the way that we need.

http://llvm-reviews.chandlerc.com/D2200