EXPERIMENTAL: clang-format on top of pending PR #3702

[rwgk]

Helper/scratch PR for testing.

Some comments are probably in bad shape, especially rst-endrst.

The purpose of this PR is to see if the reformatted C++ code works on all platforms.

Game plan from here:

After the mutually independent PRs

• clang-tidy readability-qualified-auto #3702
• Use member initializer #3704
• Manually applying two clang-format changes that need fix-ups for clang-tidy. #3705

are merged:

• Prepare source code comments for clang-formating (this will be a bit of time-consuming leg work).
• Main clang-format PR (pre-commit action & automatic large-scale changes).
• Add IncludeCategories to .clang-format (9057962).
• Done! (Unless I'm forgetting something.)

[rwgk]

Closing this PR, it served its purpose, testing with PRs #3702, #3704, #3705 combined, and laying out the game plan in the PR description. This documents a small milestone on the way to full clang-formating.