[DebugInfo] Consider debug label scope has an extra lexical block file

Summary: There are places where a case that debug label scope has an extra lexical block file is not considered properly. The modified test won't pass without this patch.

Reviewers: aprantl, HsiangKai

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66187

llvm-svn: 368891