Add explicit keywords.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44234 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index 0bacf0f..aa78e7c 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -123,7 +123,7 @@
     void operator=(const ConstantPlaceHolder &); // DO NOT IMPLEMENT
   public:
     Use Op;
-    ConstantPlaceHolder(const Type *Ty)
+    explicit ConstantPlaceHolder(const Type *Ty)
       : ConstantExpr(Ty, Instruction::UserOp1, &Op, 1),
         Op(UndefValue::get(Type::Int32Ty), this) {
     }