[ORC-RT] Re-apply "Initial ORC Runtime directories and build..." with fixes.

This reapplies 1e1d75b1, which was reverted in ce1a4d53 due to build
failures.

The unconditional dependencies on clang and llvm-jitlink in
compiler-rt/test/orc/CMakeLists.txt have been removed -- they don't appear to
be necessary, and I suspect they're the cause of the build failures seen
earlier.