support ccache with clang-tidy
As the clang tidy builds are rather slow, it would be good to use ccache with them. Historically this wasn't possible, but since cmake 3.12 the support for wrapper compilers was improved (https://gitlab.kitware.com/cmake/cmake/merge_requests/1791). So if cmake is recent enough, then the clang-tidy build can use ccache. This would be useful to have available on gitlab runner (at least).
#2 Updated by Mark Abraham about 1 month ago
Note that since 3.6, cmake has had a native property for running clang-tidy alongside a build (https://cmake.org/cmake/help/v3.6/release/3.6.html#properties) which would be simpler than our current approach of making a wrapper compiler. That looks like it would run the compiler normally, and then run clang tidy. That's ok, but quite slow.