[CMake] Add clang-bootstrap-deps target

Having this target allows other parts of the build system to add to the bootstrap dependencies without needing to be defined before the bootstrap targets are created.

This will specifically be used connect the builtins build from the LLVM runtimes directory as a dependency of the next build stage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284648 91177308-0d34-0410-b5e6-96231b3b80d8