[frozen-multi-map] Add the ability to iterate through the internal storage of the map once we have been frozen.

The intention is that this can be used once we are no longer accumulating into
the multi-map.

Author: gottesmm

include/swift/Basic/FrozenMultiMap.h

@@ -259,6 +259,26 @@ class FrozenMultiMap {
       return !pair.second.hasValue();
     });
   }
+
+  typename VectorStorage::iterator vector_begin() {
+    assert(isFrozen() && "Can only call this in map mode");
+    return storage.begin();
+  }
+
+  typename VectorStorage::iterator vector_end() {
+    assert(isFrozen() && "Can only call this in map mode");
+    return storage.end();
+  }
+
+  typename VectorStorage::const_iterator vector_begin() const {
+    assert(isFrozen() && "Can only call this in map mode");
+    return storage.begin();
+  }
+
+  typename VectorStorage::const_iterator vector_end() const {
+    assert(isFrozen() && "Can only call this in map mode");
+    return storage.end();
+  }
 };
 
 template <typename Key, typename Value, typename Storage>