Fix bug: symbol.valueDeclaration not guaranteed to be defined (#26267)

tests/cases/compiler/readonlyAssignmentInSubclassOfClassExpression.ts

@@ -0,0 +1,6 @@
+class C extends (class {} as new () => Readonly<{ attrib: number }>) {
+    constructor() {
+        super()
+        this.attrib = 2
+    }
+}