Improve --warn-symbol-ordering.

Summary:
I originally tried to simplify code and then noticed that lld doesn't
do what it tells to the user by warn(). It says "unable to order
discarded symbol" but it actually can for sections eliminated by ICF.
With this patch, lld doesn't sort such sections.

Reviewers: jhenderson, rafael

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D44180

llvm-svn: 326911