Require members of llvm.used to be named.

The effect of llvm.used is to introduce an invisible reference, so this seems
a reasonable restriction. It will be used to provide an easy ordering of
the entries in llvm.used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183743 91177308-0d34-0410-b5e6-96231b3b80d8