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

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

idea/testData/diagnosticMessage/unusedValue.kt

@@ -0,0 +1,8 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: UNUSED_VALUE
+
+fun unusedValue(): Int {
+    var x: Int
+    x = 239
+    return 42
+}