replace NominalTypeDecl::isValueType with a direct struct test

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

include/swift/AST/Decl.h

@@ -3538,14 +3538,6 @@ public:
   /// 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()) {