Revert r241330. It compiled with Visual C++ 2013 and gcc 4.9.1 (mingw) but now fails the bots.

llvm-svn: 241335
diff --git a/clang/lib/ASTMatchers/Dynamic/VariantValue.cpp b/clang/lib/ASTMatchers/Dynamic/VariantValue.cpp
index 9d8be47..a88b707 100644
--- a/clang/lib/ASTMatchers/Dynamic/VariantValue.cpp
+++ b/clang/lib/ASTMatchers/Dynamic/VariantValue.cpp
@@ -249,7 +249,7 @@
   setUnsigned(Unsigned);
 }
 
-VariantValue::VariantValue(StringRef String) : Type(VT_Nothing) {
+VariantValue::VariantValue(const std::string &String) : Type(VT_Nothing) {
   setString(String);
 }
 
@@ -319,7 +319,7 @@
   return *Value.String;
 }
 
-void VariantValue::setString(StringRef NewValue) {
+void VariantValue::setString(const std::string &NewValue) {
   reset();
   Type = VT_String;
   Value.String = new std::string(NewValue);