Cache API: Add wp_cache_flush_runtime function

Add a new function called `wp_cache_flush_runtime` to existing caching functions found in WordPress. This function allows users to flush the runtime (in-memory) cache, without flushing the entire persistent cache. 

Props: Spacedmonkey, tillkruess, flixos90, adamsilverstein, SergeyBiryukov, barryhughes. 
Fixes: #55080.


Built from https://develop.svn.wordpress.org/trunk@52772


git-svn-id: http://core.svn.wordpress.org/trunk@52361 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/cache.php

@@ -268,6 +268,19 @@ function wp_cache_flush() {
 	return $wp_object_cache->flush();
 }
 
+/**
+ * Removes all cache items from the in-memory runtime cache.
+ *
+ * @since 6.0.0
+ *
+ * @see WP_Object_Cache::flush()
+ *
+ * @return bool True on success, false on failure.
+ */
+function wp_cache_flush_runtime() {
+	return wp_cache_flush();
+}
+
 /**
  * Closes the cache.
  *