[ADT] Delete the Twine assignment operator

This makes it slightly harder to misuse Twines.  It is still possible to
refer to destroyed temporaries with the regular constructors, though.

Patch by Marco Alesiani!

llvm-svn: 209832

llvm/include/llvm/ADT/Twine.h

@@ -182,6 +182,10 @@ namespace llvm {
       assert(isValid() && "Invalid twine!");
     }
 
+    /// Since the intended use of twines is as temporary objects, assignments
+    /// when concatenating might cause undefined behavior or stack corruptions
+    Twine &operator=(const Twine &Other) LLVM_DELETED_FUNCTION;
+
     /// isNull - Check for the null twine.
     bool isNull() const {
       return getLHSKind() == NullKind;