[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