Fix our check for "random whitespace between a \ and newline" to work

Fix our check for "random whitespace between a \ and newline" to work
with dos style newlines.  I have a trivial test for this:

// RUN: clang-cc %s -verify
#define test(x, y) \
   x ## y

but I don't know how to get svn to not change newlines and testrunner
doesn't work with dos style newlines either, so "not worth it". :)

rdar://6994000

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@73945 91177308-0d34-0410-b5e6-96231b3b80d8