Extract the actual printing of the message string into a helper

function. Doing this conveniently requires moving the word wrapping to
use a StringRef which seems generally an improvement. There is a lot
that could be simplified in the word wrapping by using StringRef that
I haven't looked at yet...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140524 91177308-0d34-0410-b5e6-96231b3b80d8