clang-format: Keep '{' of dict literals on the same line in Allman style

Before:
  void f()
  {
    [object
        someMethod:@
        { @"a" : @"b" }];
  }

After:
  void f()
  {
    [object someMethod:@{ @"a" : @"b" }];
  }

This fixes llvm.org/PR19854.