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

[Andrzej Warzyński via cfe-commits]

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banach-space wrote:

As this is a "test input" rather than a "test file", could you move it to "flang/test/Driver/Inputs"?

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