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.

llvm-svn: 199198