Clang is being really peculiar.  Fix with:

-  inline hb_blob_t *sanitize (hb_blob_t *blob) { return c->sanitize<Type> (blob); }
+  inline hb_blob_t *sanitize (hb_blob_t *blob) { return c[0].template/*clang idiosyncrasy*/sanitize_blob<Type> (blob); }