[ELF] Handle -u before input files

If both a.a and b.so define foo

```
ld.bfd -u foo a.a b.so  # foo is defined
ld.bfd a.a b.so -u foo  # foo is defined
ld.bfd -u foo b.so a.a  # foo is undefined (provided at runtime by b.so)
ld.bfd b.so a.a -u foo  # foo is undefined (provided at runtime by b.so)
```

In all cases we make foo undefined in the output.  I tend to think the
GNU ld behavior makes more sense.

* In their model, they have to treat -u as a fake object file with an
  undefined symbol before all input files, otherwise the first archive would not be fetched.
* Following their behavior allows us to drop a --warn-backrefs special case.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D81052