Do not make an unnecessary copy of DenseMap.

llvm-svn: 198035
diff --git a/lld/lib/Core/Resolver.cpp b/lld/lib/Core/Resolver.cpp
index 1a86609..b7e7095 100644
--- a/lld/lib/Core/Resolver.cpp
+++ b/lld/lib/Core/Resolver.cpp
@@ -49,7 +49,7 @@
   }
 
 private:
-  const llvm::DenseSet<const Atom *> _liveAtoms;
+  const llvm::DenseSet<const Atom *> &_liveAtoms;
 };
 
 /// This is used as a filter function to std::remove_if to coalesced atoms.