No functionality change. Use unreachable in getCompileUnit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176962 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/AsmPrinter/DIE.cpp b/lib/CodeGen/AsmPrinter/DIE.cpp
index 68d6b6a..bbb0432 100644
--- a/lib/CodeGen/AsmPrinter/DIE.cpp
+++ b/lib/CodeGen/AsmPrinter/DIE.cpp
@@ -120,7 +120,7 @@
       return p;
     p = p->getParent();
   }
-  return NULL;
+  llvm_unreachable("We should not have orphaned DIEs.");
 }
 
 #ifndef NDEBUG