Convert unsigned enum class consts to static constexpr

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

src/hb-buffer.hh

diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index ab963a5..330f88b 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -119,7 +119,7 @@
   /* Text before / after the main buffer contents.
    * Always in Unicode, and ordered outward.
    * Index 0 is for "pre-context", 1 for "post-context". */
-  enum { CONTEXT_LENGTH = 5u };
+  static constexpr unsigned CONTEXT_LENGTH = 5u;
   hb_codepoint_t context[2][CONTEXT_LENGTH];
   unsigned int context_len[2];