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

[Andrzej Warzyński via cfe-commits]

https://github.com/banach-space approved this pull request.

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 :)

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