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

The flang driver was silently ignoring the `main()` function in
`Fortran_main.a` for entry into the Fortran program unit if an external
`main()` as supplied (e.g., via cross-language linkage with Fortran and
C/C++). This PR fixes this by making sure that the linker always pulls
in the `main()` definition from `Fortran_main.a` and consequently fails
due to multiple definitions of the same symbol if another object file
also has a definition of `main()`.