[llvm-libtool-darwin] Print a warning if object file names are repeated

Print a warning if `llvm-libtool-darwin` if any of the object
files provided by the user have the same file name.

The tool will now print a warning if there is a name collision across:

* Two object files
* An object file and an object file from within a static library
* Two object files from different static libraries

Here is an example of the error:

```
$ llvm-libtool-darwin -static -o archive.a out.o out.o
error: file 'out.o' was specified multiple times.
in: out.o
in: out.o

$ llvm-libtool-darwin -static -o archive.a out.o
$ llvm-libtool-darwin -static -o combined.a archive.a out.o
error: file 'out.o' was specified multiple times.
in: archive.a
in: out.o
```

This change mimics apple's cctools libtool's behavior which always shows a warning in such cases.

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D113130