Make Destroy.Op functions able to have a body (#6729)

This is iterating on how `Destroy.Op` generates, to start adding body
capabilities. This changes the way the signature is created, and adds a
`CoreWitness` function kind so that mangling can prevent name
collisions. The result is that what _was_ `DestroyOp` is now
`Core.Destroy.Op` or, as can be seen in
toolchain/lower/testdata/interop/cpp/nullptr.carbon,
`_COp.<hash>:core.Destroy.Core` where `:core` is indicating that it's a
core witness (taking a note from `:thunk`).

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/check/cpp/operators.cpp

@@ -258,10 +258,23 @@ static auto MakeCppStdInitializerListMake(Context& context, SemIR::LocId loc_id,
   // Create a builtin function to perform the conversion from array type to
   // initializer list type. We name the synthesized function as if it were a
   // constructor of std::initializer_list.
-  return MakeBuiltinFunction(
-      context, loc_id, SemIR::BuiltinFunctionKind::CppStdInitializerListMake,
-      init_list_class.scope_id, init_list_class.name_id,
-      {.param_type_ids = {array_type_id}, .return_type_id = init_list_type_id});
+  // TODO: Find a better way to handle this. Ideally we should stop using this
+  // function entirely and declare the necessary builtin in the prelude.
+  auto [decl_id, function_id] =
+      MakeGeneratedFunctionDecl(context, loc_id,
+                                {.parent_scope_id = init_list_class.scope_id,
+                                 .name_id = init_list_class.name_id,
+                                 .param_type_ids = {array_type_id},
+                                 .return_type_id = init_list_type_id});
+
+  auto& function = context.functions().Get(function_id);
+  CARBON_CHECK(IsValidBuiltinDeclaration(
+      context, function,
+      SemIR::BuiltinFunctionKind::CppStdInitializerListMake));
+  function.SetBuiltinFunction(
+      SemIR::BuiltinFunctionKind::CppStdInitializerListMake);
+
+  return decl_id;
 }
 
 // Returns information about the Carbon signature to import when importing a C++