Revert https://github.com/ruby/ruby/pull/2486

This reverts commits: 10d6a3aca7 8ba48c1b85 fba8627dc1 dd883de5ba
6c6a25feca 167e6b48f1 7cb96d41a5 3207979278 595b3c4fdd 1521f7cf89
c11c5e69ac cf33608203 3632a812c0 f56506be0d 86427a3219 .

The reason for the revert is that we observe ABA problem around
inline method cache.  When a cache misshits, we search for a
method entry.  And if the entry is identical to what was cached
before, we reuse the cache.  But the commits we are reverting here
introduced situations where a method entry is freed, then the
identical memory region is used for another method entry.  An
inline method cache cannot detect that ABA.

Here is a code that reproduce such situation:

```ruby
require 'prime'

class << Integer
  alias org_sqrt sqrt
  def sqrt(n)
    raise
  end

  GC.stress = true
  Prime.each(7*37){} rescue nil # <- Here we populate CC
  class << Object.new; end

  # These adjacent remove-then-alias maneuver
  # frees a method entry, then immediately
  # reuses it for another.
  remove_method :sqrt
  alias sqrt org_sqrt
end

Prime.each(7*37).to_a # <- SEGV
```

1 files changed, 2 insertions, 2 deletions

diff --git a/gc.c b/gc.c
index 52b3a66b6c..c9f26cc174 100644
--- a/gc.c
+++ b/gc.c
@@ -7829,9 +7829,9 @@ void rb_update_st_references(struct st_table *ht)
 }
 
 static void
-gc_ref_update_method_entry(rb_objspace_t *objspace, const rb_method_entry_t *me)
+gc_ref_update_method_entry(rb_objspace_t *objspace, rb_method_entry_t *me)
 {
-    const rb_method_definition_t *def = me->def;
+    rb_method_definition_t *def = me->def;
 
     UPDATE_IF_MOVED(objspace, me->owner);
     UPDATE_IF_MOVED(objspace, me->defined_class);