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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> > Would it be possible to test this?
> 
> Sure thing! That's the next thing I can look at. I'd like to probe though if the general solution is acceptable.

Makes sense to me - looks like a great improvement!

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