Add explicit keywords.

llvm-svn: 43464
diff --git a/llvm/lib/Analysis/ValueNumbering.cpp b/llvm/lib/Analysis/ValueNumbering.cpp
index bdb9422..8665acd 100644
--- a/llvm/lib/Analysis/ValueNumbering.cpp
+++ b/llvm/lib/Analysis/ValueNumbering.cpp
@@ -78,7 +78,7 @@
   ///
   struct VISIBILITY_HIDDEN BVNImpl : public InstVisitor<BVNImpl> {
     std::vector<Value*> &RetVals;
-    BVNImpl(std::vector<Value*> &RV) : RetVals(RV) {}
+    explicit BVNImpl(std::vector<Value*> &RV) : RetVals(RV) {}
 
     void visitCastInst(CastInst &I);
     void visitGetElementPtrInst(GetElementPtrInst &I);