Remove the assertion for now. This breaks lld.

lld has a hashtable with StringRef keys; it needs to iterate over the keys in
*insertion* order.  This is currently implemented as std::vector<StringRef> +
DenseMap<StringRef, T>.  This will probably need a proper
DenseMapInfo<StringRef> if we don't want to lose memory/performance by
migrating to a different data structure.

llvm-svn: 187868