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

[Michael Klemm via cfe-commits]

mjklemm wrote:

> LGTM, thank you for taking care of this 🙏🏻
> 
> Dare I ask - what's "dupes"? I only found [dupe](https://dictionary.cambridge.org/dictionary/english/dupe). Also, please wait for @kiranchandramohan to approve before merging this :)

I used "dupes" in the sense of being fooled.  I can of course still change the name to something like "main_linktime_duplicate.f90" or the likes.

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