Sema: Enforce that stored properties of @_fixed_layout classes only reference public types

This is the same restriction as with @Frozen structs. Even though
classes are lowered as a single reference and clients don't usually
manipulate their stored property layout directly, @Frozen classes
can have @inlinable designated initializers.

Author: slavapestov

lib/Sema/TypeCheckAccess.cpp

@@ -1051,15 +1051,16 @@ class UsableFromInlineChecker : public AccessControlCheckerBase,
   UNINTERESTING(Accessor) // Handled by the Var or Subscript.
   UNINTERESTING(OpaqueType) // Handled by the Var or Subscript.
 
-  /// If \p VD's layout is exposed by a @frozen struct, return said struct.
+  /// If \p VD's layout is exposed by a @frozen struct or class, return said
+  /// struct or class.
   ///
-  /// Stored instance properties in @frozen structs must always use
+  /// Stored instance properties in @frozen structs and classes must always use
   /// public/@usableFromInline types. In these cases, check the access against
   /// the struct instead of the VarDecl, and customize the diagnostics.
   static const ValueDecl *
   getFixedLayoutStructContext(const VarDecl *VD) {
     if (VD->isLayoutExposedToClients())
-      return dyn_cast<StructDecl>(VD->getDeclContext());
+      return dyn_cast<NominalTypeDecl>(VD->getDeclContext());
 
     return nullptr;
   }

test/attr/attr_fixed_layout_class.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5
+
+private struct PrivateType {}
+// expected-note@-1 {{struct 'PrivateType' is not '@usableFromInline' or public}}
+// expected-note@-2 {{initializer 'init()' is not '@usableFromInline' or public}}
+// expected-note@-3 {{type declared here}}
+
+@_fixed_layout public class UsesPrivateType {
+  private var y1: PrivateType
+  // expected-error@-1 {{type referenced from a stored property in a '@frozen' struct must be '@usableFromInline' or public}}
+
+  private var y2 = PrivateType()
+  // expected-error@-1 {{struct 'PrivateType' is private and cannot be referenced from a property initializer in a '@frozen' type}}
+  // expected-error@-2 {{initializer 'init()' is private and cannot be referenced from a property initializer in a '@frozen' type}}
+  // expected-error@-3 {{type referenced from a stored property with inferred type 'PrivateType' in a '@frozen' struct must be '@usableFromInline' or public}}
+
+  init() {}
+}