Don't add irrelevant items to queue in DwarfCompileUnit::createScopeChildrenDIE (NFC)

Instead of popping them and then immediately throwing them away, we can
just filter out globals and items in different scopes before adding them
to WorkList. Shouldn't change anything but keep the queue smaller.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D113864