Mark Abseil hardening assert in AssertSameContainer as slow.

When Abseil hardening is enabled, this assertion's condition gets evaluated every time an iterator equality check is performed on iterators from a raw_hash_set.  This occurs very frequently when looping over the contents of sets, due to loop conditions like `it != end()`, and causes about 0.4% CPU overhead to some Google workloads.

PiperOrigin-RevId: 681560141
Change-Id: Ia0177cbef7cf67f9f0b6adf2cf55d9a2ed320d2d

absl/container/internal/raw_hash_set.h

@@ -1753,7 +1753,7 @@ inline void AssertSameContainer(const ctrl_t* ctrl_a, const ctrl_t* ctrl_b,
             "hashtable.");
     fail_if(true, "Comparing non-end() iterators from different hashtables.");
   } else {
-    ABSL_HARDENING_ASSERT(
+    ABSL_HARDENING_ASSERT_SLOW(
         AreItersFromSameContainer(ctrl_a, ctrl_b, slot_a, slot_b) &&
         "Invalid iterator comparison. The iterators may be from different "
         "containers or the container might have rehashed or moved. Consider "