[flang][re-apply] Fix seg fault CodeGenAction::executeAction() (#78672)

If `generateLLVMIR()` fails, we still continue using the module we
failed to generate which causes a seg fault if LLVM code-gen failed for
some reason or another. This commit fixes this issue.

Re-applies PR #78269 and adds LLVM and MLIR dependencies that were
missed in the PR. The missing libs were: `LLVMCore` & `MLIRIR`.

This reverts commit 4fc75062.