clang-format: Fix incorrect detection of ObjC "in" keyword.

Before:
  for (auto v : in [1]) { ..

After:
  for (auto v : in[1]) { ..

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224513 91177308-0d34-0410-b5e6-96231b3b80d8