[CS] Generalize implied result handling

Track the implied result exprs in the constraint
system, and allow arbitrary propagation of
implied results down if/switch expression
branches. This is required for allowing implied
results in non-single-expression closures.

lib/Sema/CSSolver.cpp

@@ -148,6 +148,10 @@ Solution ConstraintSystem::finalize() {
     (void)inserted;
   }
 
+  // Remember implied results.
+  for (auto impliedResult : ImpliedResults)
+    solution.ImpliedResults.insert(impliedResult);
+
   // Remember the opened types.
   for (const auto &opened : OpenedTypes) {
     // We shouldn't ever register opened types multiple times,
@@ -294,6 +298,10 @@ void ConstraintSystem::applySolution(const Solution &solution) {
     argumentMatchingChoices.insert(argumentMatch);
   }
 
+  // Remember implied results.
+  for (auto impliedResult : solution.ImpliedResults)
+    ImpliedResults.insert(impliedResult);
+
   // Register the solution's opened types.
   for (const auto &opened : solution.OpenedTypes) {
     OpenedTypes.insert(opened);
@@ -667,6 +675,7 @@ ConstraintSystem::SolverScope::SolverScope(ConstraintSystem &cs)
   numAppliedPropertyWrappers = cs.appliedPropertyWrappers.size();
   numResolvedOverloads = cs.ResolvedOverloads.size();
   numInferredClosureTypes = cs.ClosureTypes.size();
+  numImpliedResults = cs.ImpliedResults.size();
   numContextualTypes = cs.contextualTypes.size();
   numTargets = cs.targets.size();
   numCaseLabelItems = cs.caseLabelItems.size();
@@ -789,6 +798,9 @@ ConstraintSystem::SolverScope::~SolverScope() {
   // Remove any inferred closure types (e.g. used in result builder body).
   truncate(cs.ClosureTypes, numInferredClosureTypes);
 
+  // Remove any implied results.
+  truncate(cs.ImpliedResults, numImpliedResults);
+
   // Remove any contextual types.
   truncate(cs.contextualTypes, numContextualTypes);