[clang] [flang] [lld] [flang] Generate main only when a Fortran program statement is present (PR #89938)

[Kelvin Li via cfe-commits]

kkwli wrote:

This change will also break backward compatibility that the old object file that contain `main` can no longer use the new compiler to link with other objects. I think we can put a warning in the release note or something like that to warn users.

https://github.com/llvm/llvm-project/pull/89938