Fix regression introduced in r205566.

In r205566, I made a change to Resolver so that Resolver revisit
only archive files in --start-group and --end-group pair. That's
not correct, as it also has to revisit DSO files.

This patch is to fix the issue.

Added a test to demonstrate the fix. I confirmed that it succeeded
before r205566, failed after r205566, and is ok with this patch.

Differential Revision: http://reviews.llvm.org/D3734

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208797 91177308-0d34-0410-b5e6-96231b3b80d8