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
Modify DwarfDebug::constructImportedEntityDIE to return rather than insert into the scope

Another step towards improving lexical_scope handling

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216839 91177308-0d34-0410-b5e6-96231b3b80d8
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