Convert tag enum class consts to static constexpr

Part of https://github.com/harfbuzz/harfbuzz/issues/1553
diff --git a/src/hb-aat-layout-ankr-table.hh b/src/hb-aat-layout-ankr-table.hh
index f6f1246..236e4aa 100644
--- a/src/hb-aat-layout-ankr-table.hh
+++ b/src/hb-aat-layout-ankr-table.hh
@@ -58,7 +58,7 @@
 
 struct ankr
 {
-  enum { tableTag = HB_AAT_TAG_ankr };
+  static constexpr hb_tag_t tableTag = HB_AAT_TAG_ankr;
 
   const Anchor &get_anchor (hb_codepoint_t glyph_id,
 			    unsigned int i,