Introduce line editor library.

This library will be used by clang-query. I can imagine LLDB becoming another
client of this library, so I think LLVM is a sensible place for it to live.
It wraps libedit, and adds tab completion support.

The code is loosely based on the line editor bits in LLDB, with a few
improvements:

 - Polymorphism for retrieving the list of tab completions, based on
   the concept pattern from the new pass manager.

 - Tab completion doesn't corrupt terminal output if the input covers
   multiple lines. Unfortunately this can only be done in a truly horrible
   way, as far as I can tell. But since the alternative is to implement our
   own line editor (which I don't think LLVM should be in the business of
   doing, at least for now) I think it may be acceptable.

 - Includes a fallback for the case where the user doesn't have libedit
   installed.

Note that this uses C stdio, mainly because libedit also uses C stdio.

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

llvm-svn: 200595

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