Don't print descriptors' toString() in diagnostic messages

Use DescriptorRenderer.TEXT instead, it doesn't print the debug information

idea/testData/diagnosticMessage/extensionInClassReference.kt

@@ -0,0 +1,8 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED
+
+class extensionInClassReference {
+    fun Int.foo() {}
+
+    val bar = Int::foo
+}