[flang] [clang] [flang][Driver] Let the linker fail on multiple definitions of main() (PR #73124)

[Michael Klemm via cfe-commits]

mjklemm wrote:

> May be I missed it when reading through the comments of this PR, why there is a definition of `main()` being generated on the Fortran side when the Fortran sources are procedures or modules?

It was always generated before, but the bug was hidden by the linker doing the right thing by incidence.  I have fixed that so that such an error does no longer unnoticed.



https://github.com/llvm/llvm-project/pull/73124