clang-format: Fix bug introduced in r198871.

We cannot simply change the start column to accomodate for the @ in an
ObjC string literal as that will make clang-format happily violate the
column limit.

Use a different workaround instead. However, a better long-term
solution might be to join the @ and the rest of the literal into a
single token.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199198 91177308-0d34-0410-b5e6-96231b3b80d8