Improve diagnostic message for UNSUPPORTED_FEATURE

Render lowercase words separated by spaces instead of CamelCase in the
diagnostic message

idea/testData/diagnosticMessage/unsupportedFeature.kt

@@ -0,0 +1,7 @@
+// !DIAGNOSTICS_NUMBER: 1
+// !DIAGNOSTICS: UNSUPPORTED_FEATURE
+// LANGUAGE_VERSION: 1.0
+
+fun test() {
+    mapOf("1" to "2").forEach { (key, value) -> }
+}