Also write for all configurations from multi-config generators.
This field was added in the Clang 5 documentation and not present in the
Clang 4 documentation (sometime between Dec 2016 and Mar 2017 according
to `web.archive.org`).
compile_commands.json was being written for every permutation of
cross configurations. Deduplicate so only one command is output
for each configuration.
Fixes: #23733
