Fix layouting of single-line-comments preceded by an escaped newline.

Previously, we'd format
  int i;\
  // comment
as
  int i; // comment

The problem is that the escaped newline is part of the next token, and
thus the raw token text of the comment doesn't start with "//".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171713 91177308-0d34-0410-b5e6-96231b3b80d8