[arc] Once we have finished an iteration of ARC, clear the ImmutablePointerSetFactory.

Previously due to the way that ARC works, it was impossible to trigger any
memory safety issues. That being said the fact that the memory safety here is
non-obvious suggests that the right thing to do is just bite the bullet and
clear the ImmutablePointerSetFactory.

Author: gottesmm

include/swift/Basic/ImmutablePointerSet.h

@@ -199,6 +199,8 @@ template <typename T> class ImmutablePointerSetFactory {
   // statically.
   static PtrSet *getEmptySet() { return &EmptyPtrSet; }
 
+  void clear() { Set.clear(); }
+
   /// Given a sorted and uniqued list \p Array, return the ImmutablePointerSet
   /// containing Array. Asserts if \p Array is not sorted and uniqued.
   PtrSet *get(ArrayRef<PtrTy> Array) {

lib/SILOptimizer/ARC/ARCSequenceOpts.cpp

@@ -241,6 +241,7 @@ bool LoopARCPairingContext::processRegion(const LoopRegion *Region,
     Evaluator.clearLoopState(Region);
     Context.DecToIncStateMap.clear();
     Context.IncToDecStateMap.clear();
+    Evaluator.clearSetFactory();
 
     // This ensures we only ever recompute post dominating releases on the first
     // iteration.

lib/SILOptimizer/ARC/GlobalLoopARCSequenceDataflow.h

@@ -110,6 +110,11 @@ class LoopARCSequenceDataflowEvaluator {
   /// Remove \p I from the interesting instruction list of its parent block.
   void removeInterestingInst(SILInstruction *I);
 
+  /// Clear the folding node set of the set factory we have stored internally.
+  void clearSetFactory() {
+    SetFactory.clear();
+  }
+
 private:
   /// Merge in the BottomUp state of any successors of DataHandle.getBB() into
   /// DataHandle.getState().