[Serialization] Get SubstitutionList directly from a NameAliasType.

(cherry picked from commit b26e983)

Author: DougGregor

include/swift/AST/Types.h

@@ -1617,12 +1617,6 @@ class NameAliasType final
     return getNumSubstitutions();
   }
 
-  /// Retrieve the set of substitutions to be applied to the declaration to
-  /// produce the underlying type.
-  SubstitutionList getSubstitutionList() const {
-    return {getTrailingObjects<Substitution>(), getNumSubstitutions()};
-  }
-
   /// Retrieve the generic signature used for substitutions.
   GenericSignature *getGenericSignature() const {
     return getNumSubstitutions() > 0
@@ -1648,6 +1642,12 @@ class NameAliasType final
                                              : Type();
   }
 
+  /// Retrieve the set of substitutions to be applied to the declaration to
+  /// produce the underlying type.
+  SubstitutionList getSubstitutionList() const {
+    return {getTrailingObjects<Substitution>(), getNumSubstitutions()};
+  }
+
   /// Retrieve the substitution map applied to the declaration's underlying
   /// to produce the described type.
   SubstitutionMap getSubstitutionMap() const;

lib/Serialization/Serialization.cpp

@@ -3631,10 +3631,7 @@ void Serializer::writeType(Type ty) {
                              addTypeRef(alias->getParent()),
                              addTypeRef(alias->getSinglyDesugaredType()));
     // Write the set of substitutions.
-    SmallVector<Substitution, 4> flatSubs;
-    if (auto genericSig = typeAlias->getGenericSignature())
-      genericSig->getSubstitutions(alias->getSubstitutionMap(), flatSubs);
-    writeSubstitutions(flatSubs, DeclTypeAbbrCodes);
+    writeSubstitutions(alias->getSubstitutionList(), DeclTypeAbbrCodes);
     break;
   }