vm_args.c (rb_warn_check): Use iseq_unique_id instead of its pointer

(This is the second try of 036bc1da6c6c9b0fa9b7f5968d897a9554dd770e.)

If iseq is GC'ed, the pointer of iseq may be reused, which may hide a
deprecation warning of keyword argument change.

http://ci.rvm.jp/results/trunk-test1@phosphorus-docker/2474221

```
1) Failure:
TestKeywordArguments#test_explicit_super_kwsplat [/tmp/ruby/v2/src/trunk-test1/test/ruby/test_keyword.rb:549]:
--- expected
+++ actual
@@ -1 +1 @@
-/The keyword argument is passed as the last hash parameter.* for `m'/m
+""
```

This change ad-hocly adds iseq_unique_id for each iseq, and use it
instead of iseq pointer.  This covers the case where caller is GC'ed.
Still, the case where callee is GC'ed, is not covered.

But anyway, it is very rare that iseq is GC'ed.  Even when it occurs, it
just hides some warnings.  It's no big deal.

1 files changed, 2 insertions, 0 deletions

diff --git a/vm_core.h b/vm_core.h
index f7ec156cc5..4b1b9e43d0 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -447,6 +447,8 @@ struct rb_iseq_constant_body {
     long unsigned total_calls; /* number of total calls with `mjit_exec()` */
     struct rb_mjit_unit *jit_unit;
 #endif
+
+    unsigned long iseq_unique_id; /* -- Remove In 3.0 -- */
 };
 
 /* T_IMEMO/iseq */