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.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
index 885be61..63cf62c 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
@@ -687,8 +687,9 @@
 }
 
 void DwarfCompileUnit::emitHeader(const MCSymbol *ASectionSym) const {
-  // Emit the compile units header.
-  Asm->OutStreamer.EmitLabel(LabelBegin);
+  // Don't bother labeling the .dwo unit, as its offset isn't used.
+  if (!Skeleton)
+    Asm->OutStreamer.EmitLabel(LabelBegin);
 
   DwarfUnit::emitHeader(ASectionSym);
 }