Implement `raw_hash_set::count()` in terms of `contains()`.

This improves generated code only in non-opt mode (debug and hardened builds) by avoiding iterator comparison assertions and stack spills, saving ~40 bytes per call site. In optimized mode (`-c opt`), this is neutral as `find(key) == end()` and `contains(key)` compile to identical assembly.

PiperOrigin-RevId: 1004966471

absl/container/internal/raw_hash_set.h

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index ecfcb5e..0b2d03d 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -3444,7 +3444,7 @@
   //   s.count("abc");
   template <class K = key_type>
   size_t count(const key_arg<K>& key) const {
-    return find(key) == end() ? 0 : 1;
+    return contains(key) ? 1 : 0;
   }
 
   // Issues CPU prefetch instructions for the memory needed to find or insert