Implement correctly overriding dependent virtual functions (#5804)

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/sem_ir/function.cpp

@@ -64,6 +64,31 @@ auto GetCalleeFunction(const File& sem_ir, InstId callee_id,
   return result;
 }
 
+auto DecomposeVirtualFunction(const File& sem_ir, InstId fn_decl_id,
+                              SpecificId base_class_specific_id)
+    -> DecomposedVirtualFunction {
+  // Remap the base's vtable entry to the appropriate constant usable in
+  // the context of the derived class (for the specific for the base
+  // class, for instance).
+  fn_decl_id = sem_ir.constant_values().GetInstId(
+      GetConstantValueInSpecific(sem_ir, base_class_specific_id, fn_decl_id));
+  auto specific_id = SemIR::SpecificId::None;
+  auto callee_id = fn_decl_id;
+  if (auto specific_function =
+          sem_ir.insts().TryGetAs<SemIR::SpecificFunction>(fn_decl_id)) {
+    specific_id = specific_function->specific_id;
+    callee_id = specific_function->callee_id;
+  }
+
+  // Identify the function we're calling by its type.
+  auto fn_type_inst =
+      sem_ir.types().GetAsInst(sem_ir.insts().Get(callee_id).type_id());
+
+  return {.fn_decl_id = fn_decl_id,
+          .function_id = fn_type_inst.As<FunctionType>().function_id,
+          .specific_id = specific_id};
+}
+
 auto Function::GetParamPatternInfoFromPatternId(const File& sem_ir,
                                                 InstId pattern_id)
     -> std::optional<ParamPatternInfo> {