Use _isWholeArchive to silence -Wunused-private-field warning.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190329 91177308-0d34-0410-b5e6-96231b3b80d8
Rename GnuLDInputGraph.h -> GnuLdInputGraph.h for consistency with other files.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190332 91177308-0d34-0410-b5e6-96231b3b80d8