Add initial callgraph support.

Using the two call interfaces, CallOpInterface and CallableOpInterface, this change adds support for an initial multi-level CallGraph. This call graph builds a set of nodes for each callable region, and connects them via edges. An edge may be any of the following types:
* Abstract
  - An edge not produced by a call operation, used for connecting to internal nodes from external nodes.
* Call
  - A call edge is an edge defined via a call-like operation.
* Child
  - This is an artificial edge connecting nested callgraph nodes.

This callgraph will be used, and improved upon, to begin supporting more interesting interprocedural analyses and transformation. In a followup, this callgraph will be used to support more complex inlining support.

PiperOrigin-RevId: 270724968