Nicer display of unprintable source, and fix caret display for non-ascii text

Unprintable source in diagnostics is transformed to a printable form and then
displayed with reversed colors if possible. Unprintable characters are
displayed as <U+NNNN> while bytes that do not represent valid characters are
shown as <XX>.

Column adjustments to diagnostic carets, highlighted ranges, and fixups are
made both for characters escaped as above and for characters which are
printable but take up more than a single column.

llvm-svn: 154946