fix generated_body_to_codeinfo to avoid `string` (which is not always defined) (#57925)
Fixes a discrepancy between the code in C before #57230 and in Julia
afterwards, making sure to sequence these method definitions correctly.
Not sure how to write a reliable test since it is specific to when this
generated function is defined relative to the helpers used by this
thunk, but the issue/fix is visible with:
```
$ ./julia -e 'code_lowered(ntuple, (Returns{Nothing}, Val{1000000}))'
```
Fix #57301
(cherry picked from commit a3c48d742779fe3b866911b7477afe88d082c643)