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 +215,18 @@ namespace llvm {
       return Data[Index];
     }
 
+    /// Disallow accidental assignment from a temporary std::string.
+    StringRef &operator=(std::string &&Str) = delete;
+
+    /// Disambiguate assignment from const char *.
+    ///
+    /// This assumes the argument is a valid null-terminated C string and that
+    /// the memory will not be freed while the StringRef is in use.
+    StringRef &operator=(const char *Str) {
+      *this = StringRef(Str);
+      return *this;
+    }
+
     /// @}
     /// @name Type Conversions
     /// @{