Delay adding imported entity DIEs to the lexical scope, streamlining the check for "this scope has nothing in it"

This makes the emptiness of the scope with regards to variables and
nested scopes is the same as with regards to imported entities. Just
check if we had nothing at all before we build the node.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216840 91177308-0d34-0410-b5e6-96231b3b80d8