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

[Pete Steinfeld via cfe-commits]

psteinfeld wrote:

@mjklemm, after this change was integrated, the test Driver/ctofortran no longer succeeds.  This test gets run when you call `make check-flang`.  I'm not sure why the pre- and post-build checks did not run check-flang.

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