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.
APIs for contextual profiling. `ContextNode` is the call context-specific counter buffer. `ContextRoot` is associated to those functions that constitute roots into interesting call graphs, and is the object on which we hang off `Arena`s for allocating `ContextNode`s, as well as the `ContextNode` corresponding to such functions. Graphs of `ContextNode`s are accessible by one thread at a time.
(Tracking Issue: #89287, more details in the RFC referenced there)
