Revert "[ORC-RT] Initial ORC Runtime directories and build system files."

Some builders failed with a missing clang dependency. E.g.

CMake Error at /Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build \
  /lib/cmake/llvm/AddLLVM.cmake:1786 (add_dependencies):
The dependency target "clang" of target "check-compiler-rt" does not exist.

Reverting while I investigate.

This reverts commit 1e1d75b1.