AST: Witness stores SubstitutionList instead of SubstitutionMap

This simplifies serialization and removes some calls of
SubstitutionMap::addSubstitution() and addConformance().

lib/AST/ProtocolConformance.cpp

@@ -32,13 +32,14 @@ using namespace swift;
 
 Witness::Witness(ValueDecl *decl, SubstitutionList substitutions,
                  GenericEnvironment *syntheticEnv,
-                 SubstitutionMap reqToSynthesizedEnvMap) {
+                 SubstitutionList reqToSynthesizedEnvSubs) {
   auto &ctx = decl->getASTContext();
 
   auto declRef = ConcreteDeclRef(ctx, decl, substitutions);
   auto storedMem = ctx.Allocate(sizeof(StoredWitness), alignof(StoredWitness));
   auto stored = new (storedMem)
-      StoredWitness{declRef, syntheticEnv, std::move(reqToSynthesizedEnvMap)};
+      StoredWitness{declRef, syntheticEnv,
+                    ctx.AllocateCopy(reqToSynthesizedEnvSubs)};
   ctx.addDestructorCleanup(*stored);
 
   storage = stored;