Prep-work for clang-format.

[rwgk]

Prep-work for global clang-format (see experimental/demo PRs #3074, #3082).

NO bulk changes in this PR, just minimal changes such that a global clang-format is stable after a single pass.

This was several hours of tedious work running clang-format & clang-tidy until both are stable. Breaking out as a separate PR so that these manual changes do not have to be maintained separately while we decide how to integrate clang-format or clang-format-diff into the pre-commit hooks.

Notable steps:

• Systematically adding // clang-format off, on around all \rst\endrst blocks. — Possibly some could be omitted, but for blocks that are deliberately formatted as \rst it is safer to systematically exclude them from clang-format. Note that the sphinx error messages are often completely useless if something goes wrong, therefore letting clang-format loose on \rst blocks is likely to turn into time-consuming surprises.

• Removing stray semicolons (discovered by running clang-format v12 followed by tools/check-style.sh).

• Manually moving clang-tidy // NOLINT and // NOLINTNEXTLINE comments so that clang-format does not move them to the wrong places. (This was the most tedious part.)

• Manually reformatting comments related to static_asserts so that clang-format does not need two passes.

• git diff -U0 --no-color HEAD^ | python3 $HOME/clone/llvm-project/clang/tools/clang-format/clang-format-diff.py -p1 -style=file -i

Suggested changelog entry:

Prep-work for adding clang-format to the pre-commit hooks.

[Skylion007]

Wouldn't disabling reflow_comments be better? Or there may be some of there might be some way to get it to recognize \rst \endrst as off and on clang-format comments.

[rwgk]

This one I will basically have to do from scratch after #3087 is done. At that point the only remaining clang-format related question should be how to deal with the \rst blocks.

[rwgk]

This PR is obsolete. The only step left to do on top of the merged #3087 is somehow dealing with the \rst\endrst comment blocks. I have a simple script that inserts // clang-format off, on automatically, used for the experimental/demo draft PR #3074. There is no need for a manual step as far as I can predict, therefore this PR is obsolete.