clang-format: Prevent ObjC code from confusing the braced-init detection

This was leading to bad formatting, e.g.:
Before:
  f(^{
      @autoreleasepool {
        if (a) {
          g();
  }
  }
  });

After:
  f(^{
      @autoreleasepool {
        if (a) {
          g();
        }
      }
  });

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203777 91177308-0d34-0410-b5e6-96231b3b80d8