generalize the "end of line" checking logic to stop at any \0 at the

generalize the "end of line" checking logic to stop at any \0 at the
end of line instead of just the end of buffer.  Scratch buffers contain
embedded \0's between tokens which are logic line separators.  If a
normal text buffer contains \0's, it doesn't make a lot of sense to include
them in the caret diag output anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66374 91177308-0d34-0410-b5e6-96231b3b80d8