[dsymutil] constify trivial function.

llvm-svn: 229180
diff --git a/llvm/tools/dsymutil/DwarfLinker.cpp b/llvm/tools/dsymutil/DwarfLinker.cpp
index 7a148e2..be6fb3ad 100644
--- a/llvm/tools/dsymutil/DwarfLinker.cpp
+++ b/llvm/tools/dsymutil/DwarfLinker.cpp
@@ -34,7 +34,7 @@
     Info.resize(OrigUnit.getNumDIEs());
   }
 
-  DWARFUnit &getOrigUnit() { return OrigUnit; }
+  DWARFUnit &getOrigUnit() const { return OrigUnit; }
 
   DIEInfo &getInfo(unsigned Idx) { return Info[Idx]; }
   const DIEInfo &getInfo(unsigned Idx) const { return Info[Idx]; }