This adds the basic ability to build Clang with cmake on both Windows (MSVC), and Linux. The default options work as the current getClangCMakeBuilder providing Linux builds.
CMake generators can be selected. Ninja, MSVC, and Unix Makefiles generators have been tested.
Windows and Linux builds have also been tested. Example builders for Hexagon show options for both Windows and Linux.
This might be an interim step. By adding a dictionary of llvm projects and SVN/Git parameters for fetching an LLVM based source tree, and general this becomes a generic LLVM CMake builder with the user passing a list of projects desired in the build.