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

[Tom Eccles via cfe-commits]

tblah wrote:

Simple reproducer:
main.c
```
int main(void) {
  // call fortran code
  return 0;
}
```

fortran.f90
```
function pow(a, b)
  real :: a, b, pow
  pow = a ** b
end function
```

```
clang -c main.c -o main.o
flang-now -c fortran.f90 -o fortran.o
flang-new fortran.o main.o -o out
```

This can be built with gfortran

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