Make StringSaver::save less ambiguous.

Previously, an expression such as Saver.save(std::string("foo") + "bar")
didn't compile because there is an ambiguity as to whether the argument
is of const Twine& or StringRef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301512 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/StringSaver.h

@@ -26,7 +26,7 @@ class StringSaver final {
   StringRef save(const char *S) { return save(StringRef(S)); }
   StringRef save(StringRef S);
   StringRef save(const Twine &S) { return save(StringRef(S.str())); }
-  StringRef save(std::string &S) { return save(StringRef(S)); }
+  StringRef save(const std::string &S) { return save(StringRef(S)); }
 };
 }
 #endif