[clang] [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?

Sure thing!  That's the next thing I can look at.  I'd like to probe though if the general solution is acceptable.

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