[flang] [clang] Revert "[flang][Driver] Let the linker fail on multiple definitions of main()" (PR #74120)

[Tom Eccles via cfe-commits]

tblah wrote:

For what it's worth, `armflang` follows gfortran's behavior.

As there isn't consensus amongst fortran compilers, I'm unsure which behavior we should follow. What do other's think?

https://github.com/llvm/llvm-project/pull/74120