Mark zend_visibility_string as returning a const char* (#19951)

This is because the char* returned is immutable and should not be modified.

Author: Girgias

Zend/zend_compile.h

@@ -443,7 +443,7 @@ static zend_always_inline uint32_t zend_visibility_to_set_visibility(uint32_t vi
 // Must not clash with ZEND_SHORT_CIRCUITING_CHAIN_MASK
 #define ZEND_JMP_NULL_BP_VAR_IS 4
 
-char *zend_visibility_string(uint32_t fn_flags);
+const char *zend_visibility_string(uint32_t fn_flags);
 
 #define ZEND_PROPERTY_HOOK_COUNT 2
 #define ZEND_PROPERTY_HOOK_STRUCT_SIZE (sizeof(zend_function*) * ZEND_PROPERTY_HOOK_COUNT)

Zend/zend_inheritance.c

@@ -200,7 +200,7 @@ static void do_inherit_parent_constructor(zend_class_entry *ce) /* {{{ */
 }
 /* }}} */
 
-char *zend_visibility_string(uint32_t fn_flags) /* {{{ */
+const char *zend_visibility_string(uint32_t fn_flags) /* {{{ */
 {
 	if (fn_flags & ZEND_ACC_PUBLIC) {
 		return "public";