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

[Michael Klemm via cfe-commits]

mjklemm wrote:

> Would it be possible to test this?

I have added a simple test.  It checks that the linker indeed fails, but does not check the actual linker error message.  PLease let me know if that's desired.  I could then provide the error messages for LD and LLD.  For other environments, I'd have to rely on the community to help me getting the proper linker error messages for the test.

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