Refactor constructImportedEntityDIE(DwarfUnit, DIImportedEntity) to return a DIE rather than inserting it into a specified context.

First of many steps to improve lexical scope construction (to omit
trivial lexical scopes - those without any direct variables). To that
end it's easier not to create imported entities directly into the
lexical scope node, but to build them, then add them if necessary.