[llvm-lib] Correctly handle .lib input files

If archive files are passed as input files, llvm-lib needs to append
the members of the input archive files to the output file. This patch
implements that behavior.

This patch splits an existing function into smaller functions.
Effectively, the new code is only `if (Magic == file_magic::archive)
{ ... }` part.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32674

Differential Revision: https://reviews.llvm.org/D68204

llvm-svn: 373424