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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216838 91177308-0d34-0410-b5e6-96231b3b80d8