Delay the validaton of storage accessors until finalization.

The base mutability of storage is part of the signature, so be sure
to compute that during validation.  Also, serialize it as part of
the storage declaration, and fix some places that synthesize
declarations to set it correctly.

lib/Serialization/Serialization.cpp

@@ -2894,6 +2894,8 @@ void Serializer::writeDecl(const Decl *D) {
                           var->isStatic(),
                           getRawStableVarDeclSpecifier(var->getSpecifier()),
                           var->hasNonPatternBindingInit(),
+                          var->isGetterMutating(),
+                          var->isSetterMutating(),
                           (unsigned) accessors.Kind,
                           addTypeRef(ty),
                           addDeclRef(accessors.Get),
@@ -3047,6 +3049,8 @@ void Serializer::writeDecl(const Decl *D) {
                                 contextID,
                                 subscript->isImplicit(),
                                 subscript->isObjC(),
+                                subscript->isGetterMutating(),
+                                subscript->isSetterMutating(),
                                 (unsigned) accessors.Kind,
                                 addGenericEnvironmentRef(
                                             subscript->getGenericEnvironment()),