DIBuilder maintains this invariant and the current DwarfDebug code could
end up doing weird things if it contained declarations (such as putting
the definition DIE inside a CU that contained the declaration - this
doesn't seem like a good idea, so rather than adding logic to handle
this case we'll just ban in for now & cross that bridge if we come to
it later).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209004 91177308-0d34-0410-b5e6-96231b3b80d8
33b37c7b12