allow @_moveOnly only on structs or enums

Per the existing proposal, we're not supporting
move-only reference types right now.

Author: kavon

include/swift/AST/DiagnosticsSema.def

@@ -6704,7 +6704,7 @@ ERROR(concurrency_task_to_thread_model_global_actor_annotation,none,
       (TypeRepr*, StringRef))
 
 ERROR(moveOnly_not_allowed_here,none,
-      "'moveOnly' may only be applied to classes, structs, and enums", ())
+      "'moveOnly' only applies to structs or enums", ())
 ERROR(move_expression_not_passed_lvalue,none,
       "'move' can only be applied to lvalues", ())
 ERROR(borrow_expression_not_passed_lvalue,none,

lib/Sema/TypeCheckAttr.cpp

@@ -2096,7 +2096,7 @@ void AttributeChecker::visitMoveOnlyAttr(MoveOnlyAttr *attr) {
     return;
   }
 
-  if (isa<NominalTypeDecl>(D))
+  if (isa<StructDecl>(D) || isa<EnumDecl>(D))
     return;
 
   diagnose(attr->getLocation(), diag::moveOnly_not_allowed_here)

test/Sema/moveonly_decl_attr.swift

@@ -2,8 +2,7 @@
 
 import Swift
 
-@_moveOnly
-class C {
+@_moveOnly class C { // expected-error {{'moveOnly' only applies to structs or enums}}{{1-12=}}
     @_moveOnly // expected-error {{'@_moveOnly' attribute cannot be applied to this declaration}}
     func foo() {}
 }
@@ -21,3 +20,7 @@ enum E {
 }
 
 @_moveOnly let l = C()  // expected-error {{'@_moveOnly' attribute cannot be applied to this declaration}}
+
+@_moveOnly protocol P {} // expected-error {{'moveOnly' only applies to structs or enums}}{{1-12=}}
+@_moveOnly actor A {} // expected-error {{'moveOnly' only applies to structs or enums}}{{1-12=}}
+@_moveOnly extension C {} // expected-error {{'@_moveOnly' attribute cannot be applied to this declaration}}{{1-12=}}