Add explicit keywords.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64915 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/TableGen/AsmWriterEmitter.cpp b/utils/TableGen/AsmWriterEmitter.cpp
index e2cd5dc..93d67e8 100644
--- a/utils/TableGen/AsmWriterEmitter.cpp
+++ b/utils/TableGen/AsmWriterEmitter.cpp
@@ -48,7 +48,7 @@
     // To make VS STL happy
     AsmWriterOperand():OperandType(isLiteralTextOperand) {}
 
-    AsmWriterOperand(const std::string &LitStr)
+    explicit AsmWriterOperand(const std::string &LitStr)
       : OperandType(isLiteralTextOperand), Str(LitStr) {}
 
     AsmWriterOperand(const std::string &Printer, unsigned OpNo,