clang-format: Keep empty lines and format 1-line nested blocks.

Let clang-format consistently keep up to one empty line (configured via
FormatStyle::MaxEmptyLinesToKeep) in nested blocks, e.g. lambdas. Also,
actually format single statements in nested blocks.

Before:
  DEBUG({ int     i; });
  DEBUG({
    int i;
    // an empty line here would just be removed.
    int j;
  });

After:
  DEBUG({ int i; });
  DEBUG({
    int i;

    int j;
  });