[flang] Revert "Revert "[flang] Fix seg fault `CodeGenAction::executeAction()… (PR #78672)

[Andrzej Warzyński via flang-commits]

banach-space wrote:

Thanks for the quick fix!

[nit] "Revert "Revert "[flang] Fix seg fault `CodeGenAction::executeAction()…" --> "[flang][re-apply] Fix seg fault CodeGenAction::executeAction()" or something similar. And then copy the summary from https://github.com/llvm/llvm-project/pull/78269 with some extra notes on the delta between this and the original patch. This way folks can ignore the original patch when scanning the history. It's just a nit, so feel free to ignore :)

https://github.com/llvm/llvm-project/pull/78672