replace NominalTypeDecl::isValueType with a direct struct test

enums don't and will never have stored properties, so just be direct.

Author: kavon

include/swift/AST/Decl.h

@@ -3538,14 +3538,6 @@ class NominalTypeDecl : public GenericTypeDecl, public IterableDeclContext {
   /// Whether this nominal type qualifies as any actor (plain or distributed).
   bool isAnyActor() const;
 
-  /// Whether this nominal type corresponds to a language-level value type
-  /// (i.e., structs and enums), meaning that a shallow copy is created when
-  /// reassigning an instance member of the type. That is in contrast with
-  /// reference types like classes and actors, where such mutations are
-  /// reflected for all instance holders. A protocol is not considered a
-  /// value type, even though their conformers might be a value type.
-  bool isValueType() const;
-
   /// Return the range of semantics attributes attached to this NominalTypeDecl.
   auto getSemanticsAttrs() const
       -> decltype(getAttrs().getSemanticsAttrs()) {

lib/AST/Decl.cpp

@@ -4286,10 +4286,6 @@ bool NominalTypeDecl::isAnyActor() const {
   return isActor() || isDistributedActor();
 }
 
-bool NominalTypeDecl::isValueType() const {
-  return isa<StructDecl>(this) || isa<EnumDecl>(this);
-}
-
 GenericTypeDecl::GenericTypeDecl(DeclKind K, DeclContext *DC,
                                  Identifier name, SourceLoc nameLoc,
                                  ArrayRef<InheritedEntry> inherited,

lib/Sema/TypeCheckAttr.cpp

@@ -5629,8 +5629,8 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
           return;
         }
 
-        // 'nonisolated' is redundant for the stored properties of a value type.
-        if (nominal->isValueType() && !var->isStatic() &&
+        // 'nonisolated' is redundant for the stored properties of a struct.
+        if (isa<StructDecl>(nominal) && !var->isStatic() &&
             var->isOrdinaryStoredProperty()) {
           diagnoseAndRemoveAttr(attr, diag::nonisolated_storage_value_type,
                                 nominal->getDescriptiveKind())

lib/Sema/TypeCheckConcurrency.cpp

@@ -356,10 +356,10 @@ GlobalActorAttributeRequest::evaluate(
         return None;
       }
 
-      // ... and not if it's the instance storage of a value type
+      // ... and not if it's the instance storage of a struct
       if (!var->isStatic() && var->isOrdinaryStoredProperty()) {
         if (auto *nominal = var->getDeclContext()->getSelfNominalTypeDecl()) {
-          if (nominal->isValueType()) {
+          if (isa<StructDecl>(nominal)) {
 
             var->diagnose(diag::global_actor_on_storage_of_value_type,
                           var->getName(), nominal->getDescriptiveKind())
@@ -3626,12 +3626,12 @@ ActorIsolation ActorIsolationRequest::evaluate(
 
     case ActorIsolation::GlobalActorUnsafe:
     case ActorIsolation::GlobalActor: {
-      // Stored properties of a value type don't need global-actor isolation.
+      // Stored properties of a struct don't need global-actor isolation.
       if (auto *var = dyn_cast<VarDecl>(value))
         if (!var->isStatic() && var->isOrdinaryStoredProperty())
           if (auto *varDC = var->getDeclContext())
             if (auto *nominal = varDC->getSelfNominalTypeDecl())
-              if (nominal->isValueType())
+              if (isa<StructDecl>(nominal))
                 return ActorIsolation::forUnspecified();
 
       auto typeExpr = TypeExpr::createImplicit(inferred.getGlobalActor(), ctx);