FIR checker: introduce member function checker

compiler/testData/diagnostics/tests/enum/AbstractInEnum.fir.kt

@@ -25,10 +25,10 @@ enum class MyEnum() {
     abstract val e3: Int = <!ABSTRACT_PROPERTY_WITH_INITIALIZER!>0<!>; <!ABSTRACT_PROPERTY_WITH_GETTER!>get() = a<!>
 
     //methods
-    fun f()
+    <!NON_ABSTRACT_FUNCTION_WITH_NO_BODY!>fun f()<!>
     fun g() {}
     abstract fun h()
-    abstract fun j() {}
+    <!ABSTRACT_FUNCTION_WITH_BODY!>abstract<!> fun j() {}
 
     //property accessors
     var i: Int                       abstract get  abstract set