Create bindings for simple C++ functions

This CL wires in the IR and RsSrcCodeGen with the rest of the tool, using C++
functions as an example.

PiperOrigin-RevId: 390312876
diff --git a/rs_bindings_from_cc/rs_src_code_gen.rs b/rs_bindings_from_cc/rs_src_code_gen.rs
index 98f152d..473f908 100644
--- a/rs_bindings_from_cc/rs_src_code_gen.rs
+++ b/rs_bindings_from_cc/rs_src_code_gen.rs
@@ -95,6 +95,7 @@
         let mangled_name = &func.mangled_name;
         let ident = make_ident(&func.identifier.identifier);
         let thunk_ident = format_ident!("__rust_thunk__{}", &func.identifier.identifier);
+        // TODO(hlopko): do not emit `-> ()` when return type is void, it's implicit.
         let return_type_name = make_ident(&func.return_type.rs_name);
 
         let param_idents =