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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

>  @DavidTruby suggested only linking Fortran_main if the fortran source contains a program statement.

That would be ideal, but how is the driver meant to know that? ;-)

> One could build a mixed source project now by linking using clang and manually specifying to link the rest of the fortran runtime library. I don't think this is a good option (...)

Agreed.

> This can be built with gfortran

Where does GFortran take it's main from? IIRC, it uses a completely different mechanism.

@mjklemm any thoughts? I'm leaning towards reverting this for now - it's not obvious to me what the solution should be.

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