ruby: Fix object cache lookups on 32-bit platforms

#13204 refactored the
Ruby object cache to use a key of `LL2NUM(key_val)` instead of
`LL2NUM(key_val >> 2)`. On 32-bit systems, LL2NUM(key_val) returns
inconsistent results because a large value has to be stored as a
Bignum on the heap. This causes cache lookups to fail.

This commit restores the previous behavior of using
`ObjectCache_GetKey`, which discards the lower 2 bits, which are zero.
This enables a key to be stored as a Fixnum on both 32 and 64-bit
platforms.

Closes #13481

ruby/ext/google/protobuf_c/protobuf.c

@@ -276,17 +276,24 @@ static void ObjectCache_Init(VALUE protobuf) {
   rb_const_set(protobuf, rb_intern("SIZEOF_VALUE"), INT2NUM(SIZEOF_VALUE));
 }
 
-VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
+static VALUE ObjectCache_GetKey(const void *key) {
   VALUE key_val = (VALUE)key;
   PBRUBY_ASSERT((key_val & 3) == 0);
-  return rb_funcall(weak_obj_cache, item_try_add, 2, LL2NUM(key_val), val);
+  // Ensure the key can be stored as a Fixnum on a 32-bit system
+  VALUE new_key = LL2NUM(key_val >> 2);
+  PBRUBY_ASSERT(FIXNUM_P(new_key));
+  return new_key;
+}
+
+VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
+  VALUE key_val = ObjectCache_GetKey(key);
+  return rb_funcall(weak_obj_cache, item_try_add, 2, key_val, val);
 }
 
 // Returns the cached object for this key, if any. Otherwise returns Qnil.
 VALUE ObjectCache_Get(const void *key) {
-  VALUE key_val = (VALUE)key;
-  PBRUBY_ASSERT((key_val & 3) == 0);
-  return rb_funcall(weak_obj_cache, item_get, 1, LL2NUM(key_val));
+  VALUE key_val = ObjectCache_GetKey(key);
+  return rb_funcall(weak_obj_cache, item_get, 1, key_val);
 }
 
 /*