Replace enum with struct to avoid polluting sycl::

Author: Pennycook

sycl/doc/extensions/proposed/sycl_ext_oneapi_named_sub_group_sizes.asciidoc

@@ -163,9 +163,9 @@ namespace ext {
 namespace oneapi {
 namespace experimental {
 
-enum named_sub_group_size : uint32_t {
-  primary = /* implementation-defined */,
-  automatic = /* implementation-defined */,
+struct named_sub_group_size {
+  static constexpr uint32_t primary = /* unspecified */,
+  static constexpr uint32_t automatic = /* unspecified */,
 };
 
 inline constexpr sub_group_size_key::value_t<named_sub_group_size::primary> sub_group_size_primary;