Clang: Add support for windows target on case sensitive fs

This change adds a new CMake variable CMAKE_CLANG_VFS_OVERLAY to simplify using a VFS to compile windows applications on a case sensitive host.

The command split token for the llvm_rc command line mode, changed to ++ to disambiguate the clang-cl source separation token --.

Fixes: #20658
Topic-rename: clang-cl-vfs