Avoid undefined behavior when vector is empty.

Found by ubsan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255258 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SetVector.h

@@ -58,6 +58,8 @@ public:
     insert(Start, End);
   }
 
+  ArrayRef<T> getArrayRef() const { return vector_; }
+
   /// \brief Determine if the SetVector is empty or not.
   bool empty() const {
     return vector_.empty();

lib/Linker/LinkModules.cpp

@@ -772,8 +772,7 @@ bool ModuleLinker::run() {
       Internalize.insert(GV->getName());
   }
 
-  if (Mover.move(SrcM,
-                 makeArrayRef(&*ValuesToLink.begin(), ValuesToLink.size()),
+  if (Mover.move(SrcM, ValuesToLink.getArrayRef(),
                  [this](GlobalValue &GV, IRMover::ValueAdder Add) {
                    addLazyFor(GV, Add);
                  }))