`rb_vm_lvar_exposed()` is PUREFUNC

This function is used in `rb_vm_lvar()` and this function can be
unsed (generated into *.rbinc files automatically).
`rb_vm_lvar()` is already declared as a PUREFUNC, but
`rb_vm_lvar_exposed()` is not a PUREFUNC, so `rb_vm_lvar_exposed()`
is remained even if it is unused.

builtin.h

@@ -56,6 +56,7 @@ static inline void rb_builtin_function_check_arity13(VALUE (*f)(rb_execution_con
 static inline void rb_builtin_function_check_arity14(VALUE (*f)(rb_execution_context_t *ec, VALUE self, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE)){}
 static inline void rb_builtin_function_check_arity15(VALUE (*f)(rb_execution_context_t *ec, VALUE self, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE, VALUE)){}
 
+PUREFUNC(VALUE rb_vm_lvar_exposed(rb_execution_context_t *ec, int index));
 VALUE rb_vm_lvar_exposed(rb_execution_context_t *ec, int index);
 
 // __builtin_inline!