Remove support for splitting word-wrapped diagnostic messages on newline

characters. I could find no newline character in a diagnostic message,
and adding an assert to this code never fires in the testsuite.

I think this code is essentially dead, and was previously used for
a different purpose. If I just don't understand how it is we can end up
with a newline here please let me know (with a test case?) and I'll
revert.

llvm-svn: 140497