[llvm-ar] Support N [count] modifier

Summary:
GNU ar supports the 'N' count modifier for the extract (x) and delete (d) operations. When an archive contains multiple members with the same name, this can be used to extract (or delete) them individually. For example:

```
$ llvm-ar t archive.a
foo
foo
$ llvm-ar x archive.a
-> Writes foo twice, overwriting it the second time :( :(
$ llvm-ar xN 1 archive.a foo && mv foo foo.1
$ llvm-ar xN 2 archive.a foo && mv foo foo.2
-> Write foo twice, renaming it in between invocations to preserve all versions
```

Reviewers: ruiu, MaskRay

Reviewed By: ruiu, MaskRay

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59503

llvm-svn: 356466