Disallow StringRef assignment from temporary std::strings.

Similar to r283798, this prevents accidentally referring to temporary
storage that goes out of scope by the end of the statement:

  someStringRef = getStringByValue();
  someStringRef = (Twine("-") + otherString).str();

Note that once again the constructor still has this problem:

  StringRef someStringRef = getStringByValue();

because once again we occasionally rely on this in calls:

  takesStringRef(getStringByValue());
  takesStringRef(Twine("-") + otherString);

Still, it's a step.

Author: jrose-apple

include/llvm/ADT/StringRef.h

@@ -215,6 +215,15 @@ namespace llvm {
       return Data[Index];
     }
 
+    /// Disallow accidental assignment from a temporary std::string.
+    ///
+    /// The declaration here is extra complicated so that `stringRef = {}`
+    /// and `stringRef = "abc"` continue to select the move assignment operator.
+    template <typename T>
+    typename std::enable_if<std::is_same<T, std::string>::value,
+                            StringRef>::type &
+    operator=(T &&Str) = delete;
+
     /// @}
     /// @name Type Conversions
     /// @{