[clang-format] Fix SeparateDefinitionBlocks issues

Fixes https://github.com/llvm/llvm-project/issues/52976.

- Make no formatting for macros
- Attach comment with definition headers
- Make no change on use of empty lines at block start/end
- Fix misrecognition of keyword namespace

Differential Revision: https://reviews.llvm.org/D116663
Reviewed By: MyDeveloperDay, HazardyKnusperkeks, curdeius