[clang-tidy] insert release notes for new checkers alphabetically

Summary:
Almost all code review comments on new checkers {D55433} {D48866} {D54349} seem to ask for the release notes to be added alphabetically, plus I've seen commits by @Eugene.Zelenko reordering the lists

Make add_new_check.py add those release notes alphabetically based on checker name

If include-fixer section is seen add it at the end

Minor change in the message format to prevent double newlines added before the checker.

Do the tools themselves have unit tests? (sorry new to this game)

- Tested adding new checker at the beginning
- Tested on adding new checker in the middle
- Tested on empty ReleasesNotes.rst (as we would see after RC)

Patch by MyDeveloperDay.

Reviewers: alexfh, JonasToth, curdeius, aaron.ballman, benhamilton, hokein

Reviewed By: JonasToth

Subscribers: cfe-commits, xazax.hun, Eugene.Zelenko

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D55508

llvm-svn: 348793