Fixes use of unescaped newlines when formatting preprocessor directives.

This is the first step towards handling preprocessor directives. This
patch only fixes the most pressing issue, namely correctly escaping
newlines for tokens within a sequence of a preprocessor directive.

The next step will be to fix incorrect format decisions on #define
directives.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171393 91177308-0d34-0410-b5e6-96231b3b80d8