[InputGraph] Add capability to process Hidden nodes.

Hidden nodes could be a result of expansion, where a flavor might decide to keep
the node that we want to expand but discard it from being processed by the
resolver.

Verifies with unittests.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195516 91177308-0d34-0410-b5e6-96231b3b80d8