Remove support for splitting word-wrapped diagnostic messages on newline

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140497 91177308-0d34-0410-b5e6-96231b3b80d8