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

[Michael Klemm via cfe-commits]

mjklemm wrote:

FYI: Rebased and resolved conflict flagged by GitHub.  Alas, this means that I have lost the change to also make the linker fail on Windows.  I've sent a request to the authors of the code to perform linking on Windows to help me figure out what to do.

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