This pulls out `ContextNode` as we need to use it pretty much as-is to implement a writer. The writer will be implemented on the LLVM side because it takes a dependency on BitStreamWriter.
Since we can't reuse a header between compiler-rt and llvm, we use a header file which is copied on both sides, and test that the 2 copies are identical.
The changes adds the necessary other stuff for compiler-rt/ctx_profile testing.
