Compilation caching integration with toolchain-cas-plugin

In order to better support distributed compilation caching with Swift open source toolchains, @akyrtzi , @cachemeifyoucan and I have requested a new swiftlang repository, swiftlang/toolchain-cas-plugin. This repository will contain an implementation of the LLVM CAS plugin API to support cross-platform distributed caching in SwiftPM and other swift build systems via a shared library distributed as part of the toolchain. In order to approve the repository, the ecosystem steering group has requeste...