[region-isolation] Add isolation history support but do not wire it up to the checker.

This means that I added an IsolationHistory field to Partition. Just upstreaming
the beginning part of this work. I added some unittests to exercise the code as
well. NOTE: This means that I did need to begin tracking an
IsolationHistoryFactory and propagating IsolationHistory in the pass
itself... but we do not use it for anything.

A quick overview of the design.

IsolationHistory is the head of an immutable directed acyclic graph. It is
actually represented as an immutable linked list with a special node that ties
in extra children nodes. The users of the information are expected to get a
SmallVectorImpl and process those sibling nodes afterwards. The reason why we
use an immutable approach is that it fits well with the problem and saves space
since different partitions could be pointing at the same linked list
node. Operations occur on an isolation history by pushing/popping nodes. It is
assumed that the user will push nodes in batches with a sequence boundary at the
bottom of the addition which signals to stop processing nodes.

Tieing this together, each Partition within it contains an IsolationHistory. As
the PartitionOpEvaluator applies PartitionOps to Partition in
PartitionOpEvaluator::apply, the evaluator also updates the isolation history in
the partition by first pushing a SequenceBoundary node and then pushing nodes
that will undo the operation that it is performing. This information is used by
the method Partition::popHistory. This pops linked list nodes from its history,
performing the operation in reverse until it hits a SequenceBoundary node.

This allows for one to rewind Partition history. And if one stashes an isolation
history as a target, one can even unwind a partition to what its state was at a
specific transfer point or earlier. Once we are at that point, we can begin
going one node back at a time and see when two values that we are searching for
no longer are apart of the same region. That is a place where we want to emit a
diagnostic. We then process until we find for both of our values history points
where they were the immediate reason why the two regions merge.

rdar://123479934

Author: gottesmm

include/swift/SILOptimizer/Analysis/RegionAnalysis.h

@@ -90,7 +90,8 @@ class BlockPartitionState {
 
   BlockPartitionState(SILBasicBlock *basicBlock,
                       PartitionOpTranslator &translator,
-                      TransferringOperandSetFactory &ptrSetFactory);
+                      TransferringOperandSetFactory &ptrSetFactory,
+                      IsolationHistory::Factory &isolationHistoryFactory);
 
 public:
   bool getLiveness() const { return isLive; }
@@ -394,6 +395,8 @@ class RegionAnalysisFunctionInfo {
 
   TransferringOperandSetFactory ptrSetFactory;
 
+  IsolationHistory::Factory isolationHistoryFactory;
+
   // We make this optional to prevent an issue that we have seen on windows when
   // capturing a field in a closure that is used to initialize a different
   // field.