https://ccache.dev/manual/3.7.11.html#_configuration uses CCACHE_DIR to set
cache directory. CCACHE_PATH has a different meaning.
+ removed LLVM_CCACHE_DIR ccache argument to use CCACHE_DIR from env.
+ print additional info about ccache config and machine readable stats.
E.g.: https://reviews.llvm.org/D71197?id=233029
Will add a link to file and code to apply the patch in the next PR.
+ don't create TARGET_DIR in scripts;
+ updated section about local build;
+ partially specified inputs / outputs of scripts so it's more transparent what are the results;
+ added symlink to compile_command.json (clang-tidy will need it);
+ add IDEA files to .gitignore.
