clang-format 6 and 7 are shown by make format as valid version, but give different results when run on code in master. Meanwhile clang-format in precompiled libraries is being upgraded to 9.
- Decide and document supported versions.
- Add /* clang-format off */ or other workarounds to make result same on all supported versions.
- make format does not use clang-format from precompiled libraries on Linux, path is wrong.
clang-format version 6 to 9 should now give the same result, and that range has been marked as supported in make format. We can decide to narrow it later if needed.