middle-end/97207 - implement move assign for auto_vec<>

This implements the missing move assignment to make std::swap work
on auto_vec<>

2020-09-25  Richard Biener  <rguenther@suse.de>

	PR middle-end/97207
	* vec.h (auto_vec<T>::operator=(auto_vec<T>&&)): Implement.

gcc/vec.h

@@ -1546,7 +1546,13 @@ public:
       this->m_vec = r.m_vec;
       r.m_vec = NULL;
     }
-  void operator= (auto_vec&&) = delete;
+  auto_vec& operator= (auto_vec&& r)
+    {
+      this->release ();
+      this->m_vec = r.m_vec;
+      r.m_vec = NULL;
+      return *this;
+    }
 };