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

[Michael Klemm via cfe-commits]

mjklemm wrote:

BTW, flang legacy has this: `  -fno-fortran-main       Don't link in Fortran main` 

Adding that command line option might be the right choice.  If everyone agrees, I can see if I can get this added.

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