Convert boolean enum class consts to static constexpr

Part of https://github.com/harfbuzz/harfbuzz/issues/1553

src/hb-ot-layout.cc

diff --git a/src/hb-ot-layout.cc b/src/hb-ot-layout.cc
index 179db65..3ded3ad 100644
--- a/src/hb-ot-layout.cc
+++ b/src/hb-ot-layout.cc
@@ -1300,7 +1300,7 @@
 struct GSUBProxy
 {
   enum { table_index = 0u };
-  enum { inplace = false };
+  static constexpr bool inplace = false;
   typedef OT::SubstLookup Lookup;
 
   GSUBProxy (hb_face_t *face) :
@@ -1314,7 +1314,7 @@
 struct GPOSProxy
 {
   enum { table_index = 1u };
-  enum { inplace = true };
+  static constexpr bool inplace = true;
   typedef OT::PosLookup Lookup;
 
   GPOSProxy (hb_face_t *face) :