Make undefines check into an assertion.

Dead-strip root symbols can be undefined atoms, but should not really be
nonexistent, because dead-strip root symbols should be added to initial
undefined atoms at startup. Whenever you look up its name in the symbol
table, some type of atom will always exist.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@192831 91177308-0d34-0410-b5e6-96231b3b80d8