Misc: Use TODO() consistently in implementation stubs

#KT-13589 Fixed

idea/testData/quickfix/override/nothingToOverride/addFunctionNonUnitReturnType.kt.after

@@ -2,8 +2,7 @@
 // WITH_RUNTIME
 open class A {
     open fun f(): Int {
-        throw UnsupportedOperationException()
-    }
+        TODO("not implemented") //To change body of created functions use File | Settings | File Templates. }
 }
 class B : A() {
     <caret>override fun f(): Int = 5