Don't bother creating LabelBegin for .dwo units

This would help catch cases where we might otherwise try to reference a
dwo CU label, which would be weird - because without relocations in the
dwo file it's not generally meaningful to talk about the CU offsets
there (or, if it is, we can do so in absolute terms without using a
relocation to compute it).

llvm-svn: 221078
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
index 5f80e0c..27ed6fc 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
@@ -158,8 +158,11 @@
   void initSection(const MCSection *Section, MCSymbol *SectionSym) {
     DwarfUnit::initSection(Section);
     this->SectionSym = SectionSym;
-    LabelBegin =
-        Asm->GetTempSymbol(Section->getLabelBeginName(), getUniqueID());
+
+    // Don't bother labeling the .dwo unit, as its offset isn't used.
+    if (!Skeleton)
+      LabelBegin =
+          Asm->GetTempSymbol(Section->getLabelBeginName(), getUniqueID());
   }
 
   unsigned getLength() {