Incremental build support for the modules driver

Until recently, explicit module builds were only supported for Clang modules. With the -fmodules-driver mode upstream patch being merged, the community can finally experience explicit builds for both Clang modules discovered through module map files and C++ named modules. However, one of the main advantages of modules is their ability to being cached. Currently, -fmodules-driver does not support this. Therefore, the proposal strives towards an effort to bring forth an efficient, correct and safe filesystem cache. The proposed filesystem cache will enforce strong concurrency control. Once this cache system is fully implemented, the community will be able to use and manage it with existing Clang flags along with -fmodules-driver mode.