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

[Tom Eccles via cfe-commits]

tblah wrote:

That example is quite different because you used `program` in the fortran source. The use case here is applications which implement `main` in C then call into fortran code

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