Revert "Revert "AST: Re-enable TypeSubstituter::transformSubstitutionMap() again""

This reverts commit 47684cb.

Author: slavapestov

lib/AST/TypeSubstitution.cpp

@@ -358,7 +358,7 @@ class TypeSubstituter : public TypeTransform<TypeSubstituter> {
   std::optional<Type> transformLocalArchetypeType(LocalArchetypeType *local,
                                                   TypePosition pos);
 
-  // SubstitutionMap transformSubstitutionMap(SubstitutionMap subs);
+  SubstitutionMap transformSubstitutionMap(SubstitutionMap subs);
 
   CanType transformSILField(CanType fieldTy, TypePosition pos);
 };
@@ -474,13 +474,10 @@ Type TypeSubstituter::transformDependentMemberType(DependentMemberType *dependen
   return result;
 }
 
-// FIXME: This exposes a scalability issue; see test/SILGen/opaque_result_type_slow.swift.
-/*
 SubstitutionMap TypeSubstituter::transformSubstitutionMap(SubstitutionMap subs) {
   // FIXME: Take level into account? Move level down into IFS?
   return subs.subst(IFS);
 }
-*/
 
 CanType TypeSubstituter::transformSILField(CanType fieldTy, TypePosition pos) {
   // Type substitution does not walk into the SILBoxType's field types, because