[compiler-rt][ctx_profile] Add the instrumented contextual profiling APIs (#89838)

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)