Expand nested input elements.

Previously only the toplevel elements were expanded by expandElements().
Now we recursively call getReplacements() to expand input elements even
if they are in, say, in a group.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208144 91177308-0d34-0410-b5e6-96231b3b80d8