Fix enum case conflict in trait binding

Zend/tests/enum/gh21760.phpt

@@ -0,0 +1,19 @@
+--TEST--
+GH-21760 (Trait with class constant name conflict against enum case causes SEGV)
+--FILE--
+<?php
+
+trait X {
+    public const Up = 1;
+}
+
+enum Direction {
+    use X;
+
+    case Up;
+    case Down;
+}
+
+?>
+--EXPECTF--
+Fatal error: Direction and X define the same constant (Up) in the composition of Direction. However, the definition differs and is considered incompatible. Class was composed in %s on line %d

Zend/zend_inheritance.c

@@ -2770,6 +2770,11 @@ static bool do_trait_constant_check(
 
 	zend_class_constant *existing_constant = Z_PTR_P(zv);
 
+	if (UNEXPECTED(ZEND_CLASS_CONST_FLAGS(existing_constant) & ZEND_CLASS_CONST_IS_CASE)) {
+		emit_incompatible_trait_constant_error(ce, existing_constant, trait_constant, name, traits, current_trait);
+		return false;
+	}
+
 	if ((ZEND_CLASS_CONST_FLAGS(trait_constant) & flags_mask) != (ZEND_CLASS_CONST_FLAGS(existing_constant) & flags_mask)) {
 		emit_incompatible_trait_constant_error(ce, existing_constant, trait_constant, name, traits, current_trait);
 		return false;