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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

@kkwli and @DanielCChen - could you review https://github.com/llvm/llvm-project/pull/75816 and see whether that makes sense to you? I am referring specifically to the documentation that @mjklemm is kindly adding. Hopefully that will help folks avoid similar issues in the future.

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