Fixes PR17617: Crash on joining short if statements.

Now that we iterate on the formatting multiple times when we
have chains of preprocessor branches, we need to correctly reset
the token's previous and next pointer for the first / last token.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193071 91177308-0d34-0410-b5e6-96231b3b80d8