[clang-format] Fix DefSeparator empty line issues

- Add or remove empty lines surrounding union blocks.
- Fixes https://github.com/llvm/llvm-project/issues/53229, in which
  keywords like class and struct in a line ending with left brace or
  whose next line is left brace only, will be falsely recognized as
  definition line, causing extra empty lines inserted surrounding blocks
  with no need to be formatted.

Reviewed By: MyDeveloperDay, curdeius, HazardyKnusperkeks, owenpan
Differential Revision: https://reviews.llvm.org/D119067