Complete rewrite of interactive editing support for single- and multi-line input.

Improvements include:
* Use of libedit's wide character support, which is imperfect but a distinct improvement over ASCII-only
* Fallback for ASCII editing path
* Support for a "faint" prompt clearly distinguished from input
* Breaking lines and insert new lines in the middle of a batch by simply pressing return
* Joining lines with forward and backward character deletion
* Detection of paste to suppress automatic formatting and statement completion tests
* Correctly reformatting when lines grow or shrink to occupy different numbers of rows
* Saving multi-line history, and correctly preserving the "tip" of history during editing
* Displaying visible ^C and ^D indications when interrupting input or sending EOF
* Fledgling VI support for multi-line editing
* General correctness and reliability improvements

llvm-svn: 222163