[frozen-multi-map] Add the ability to "unfreeze" the multi-map so one can add incrementally more elements to the multi-map.

For those unfamiliar, this map is a vector of pairs that we stable sort by the
key when we "freeze" it so we can run map operations upon the keys. This allows
one to accumulate into the multi-map and then once one has finished
accumulating, perform these map operations.

While doing some work in the move checker, I thought I would need the ability to
incrementally update a multi-map by appending more entires. Due to the design,
supporting this is as simple as unfreezing the map. This results in one being no
longer able to run map operations without hitting an assert. When one freezes
again, the stable sort will put the new entries in the appropriate place in the
already sorted initial part of the array.

Turns out I didn't need this, but seemed useful, so I am upstreaming it.

Author: gottesmm

include/swift/Basic/FrozenMultiMap.h

@@ -122,7 +122,11 @@ class FrozenMultiMap {
 
   bool isFrozen() const { return frozen; }
 
-  /// Set this map into its frozen state when we
+  /// Set this map into its frozen state. This stable sorts our internal array
+  /// to create our map like context.
+  ///
+  /// After this, one can only use map like operations and non-mutable vector
+  /// operations instead of full mutable/non-mutable vector operations.
   void setFrozen() {
     std::stable_sort(storage.begin(), storage.end(),
                      [&](const std::pair<Key, Optional<Value>> &lhs,
@@ -134,6 +138,13 @@ class FrozenMultiMap {
     frozen = true;
   }
 
+  /// Unfreeze the map, so one can go back to using mutable vector
+  /// operations. After one calls this until one freezes the map again, one
+  /// cannot use map operations.
+  ///
+  /// This allows one to incrementally update the map.
+  void unfreeze() { frozen = false; }
+
   /// Reset the frozen multimap in an unfrozen state with its storage cleared.
   void reset() {
     storage.clear();