Add an explicit preprocessing step when using llvm-rc as RC file compiler
Up until now the llvm-rc compiler does not support preprocessing. This makes targeting windows with the clang toolchain unpracticable when the "rc" compiler is not present on the compilation machine. In my usage scenario, we compile windows binaries on a linux machine where the rc compiler is not available.
To work around this situation, I proposed to include an explicit preprocessing step to rc file compilation, feeding the preprocessed input to the llvm-rc compiler (Merge request 4180).
This procedure could also be used to support dependency tracking for rc files when using the ninja generator. Note that an explicit preprocessing step is run inside cmcldeps before calling the rc compiler as the rc compiler doesn't output dependencies.