Make C++ enum types impl Core.Copy. (#7013)

Remove special-case handling in conversion logic for C++ enum types,
synthesize a custom witness of `Core.Copy` using the `primitive_copy`
builtin function.

toolchain/check/function.cpp

@@ -121,7 +121,7 @@ static auto MakeFunctionSignature(Context& context, SemIR::LocId loc_id,
 
   StartFunctionSignature(context);
 
-  // Build and add a `[ref self: Self]` parameter if needed.
+  // Build and add a `self: Self` or `ref self: Self` parameter if needed.
   if (args.self_type_id.has_value()) {
     context.full_pattern_stack().StartImplicitParamList();