AST: Cache substituted substitution maps inside the InFlightSubstitution

A substitution map contains conformances, and a conformance can contain
a substitution map. This will always be a DAG, but not a tree, and to
avoid exponential blowup in certain edge cases, let's cache the work to
avoid visiting the same substitution map repeatedly, if multiple
conformances refer to the same substitution map.

Author: slavapestov

include/swift/AST/InFlightSubstitution.h

@@ -23,11 +23,14 @@
 #define SWIFT_AST_INFLIGHTSUBSTITUTION_H
 
 #include "swift/AST/SubstitutionMap.h"
+#include "llvm/ADT/DenseMap.h"
 
 namespace swift {
 class SubstitutionMap;
 
 class InFlightSubstitution {
+  friend class SubstitutionMap;
+
   TypeSubstitutionFn BaselineSubstType;
   LookupConformanceFn BaselineLookupConformance;
   SubstOptions Options;
@@ -41,6 +44,7 @@ class InFlightSubstitution {
     unsigned expansionIndex = 0;
   };
   llvm::SmallVector<ActivePackExpansion, 4> ActivePackExpansions;
+  llvm::SmallDenseMap<SubstitutionMap, SubstitutionMap, 2> SubMaps;
 
   Type projectLaneFromPackType(
       Type substType, unsigned level);

lib/AST/SubstitutionMap.cpp

@@ -325,6 +325,13 @@ SubstitutionMap SubstitutionMap::subst(TypeSubstitutionFn subs,
 SubstitutionMap SubstitutionMap::subst(InFlightSubstitution &IFS) const {
   if (empty()) return SubstitutionMap();
 
+  // FIXME: Get this caching working with pack expansions as well.
+  if (IFS.ActivePackExpansions.empty()) {
+    auto found = IFS.SubMaps.find(*this);
+    if (found != IFS.SubMaps.end())
+      return found->second;
+  }
+
   SmallVector<Type, 4> newSubs;
   for (Type type : getReplacementTypes()) {
     newSubs.push_back(type.subst(IFS));
@@ -345,7 +352,12 @@ SubstitutionMap SubstitutionMap::subst(InFlightSubstitution &IFS) const {
   }
 
   assert(oldConformances.empty());
-  return SubstitutionMap(genericSig, newSubs, newConformances);
+  auto result = SubstitutionMap(genericSig, newSubs, newConformances);
+
+  if (IFS.ActivePackExpansions.empty())
+    (void) IFS.SubMaps.insert(std::make_pair(*this, result));
+
+  return result;
 }
 
 SubstitutionMap