Convert tag enum class consts to static constexpr

Part of https://github.com/harfbuzz/harfbuzz/issues/1553
diff --git a/src/hb-ot-layout-base-table.hh b/src/hb-ot-layout-base-table.hh
index 6c39932..dd0fba1 100644
--- a/src/hb-ot-layout-base-table.hh
+++ b/src/hb-ot-layout-base-table.hh
@@ -464,7 +464,7 @@
 
 struct BASE
 {
-  enum { tableTag = HB_OT_TAG_BASE };
+  static constexpr hb_tag_t tableTag = HB_OT_TAG_BASE;
 
   const Axis &get_axis (hb_direction_t direction) const
   { return HB_DIRECTION_IS_VERTICAL (direction) ? this+vAxis : this+hAxis; }