[flang] [clang] Revert "[flang][Driver] Let the linker fail on multiple definitions of main()" (PR #74120)

[Tom Eccles via cfe-commits]

tblah wrote:

> @tblah Would you mind trying this new option with one application code to see if that will help?

Yes this flag works for me, thanks! I think it would be a shame to require a flag not needed for classic-flang or gfortran, but we can come back to this. This solution is good enough for me now. Thanks!

https://github.com/llvm/llvm-project/pull/74120