[flang] Initial lowering for empty program

This patch enable lowering from Fortran to FIR for a basic empty
program. It brings all the infrastructure needed for that. As discussed
previously, this is the first patch for lowering and follow up patches
should be smaller.

With this patch we can lower the following code:

```
program basic
end program
```

To a the FIR equivalent:

```
func @_QQmain() {
  return
}
```

Follow up patch will add lowering of more complex constructs.

Reviewed By: kiranchandramohan, schweitz, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D118436

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