fix Bug 12924

http://llvm.org/bugs/show_bug.cgi?id=12924

This issue was that the source location was pointing to a
non-printable character and so CaretEnd was pointing one
_column_ past the caret but not one _character_ past the
caret. So the conversion between column and byte locations
wasn't working (because the conversion is only valid from
the first column or byte of a character).

llvm-svn: 157372