FIR checker: introduce member function checker

compiler/testData/diagnostics/tests/AbstractInClass.fir.kt

@@ -23,10 +23,10 @@ class MyClass() {
     <!ABSTRACT_PROPERTY_IN_NON_ABSTRACT_CLASS!>abstract<!> val e3: Int = 0; get() = a
 
     //methods
-    fun f()
+    <!NON_ABSTRACT_FUNCTION_WITH_NO_BODY!>fun f()<!>
     fun g() {}
-    abstract fun h()
-    abstract fun j() {}
+    <!ABSTRACT_FUNCTION_IN_NON_ABSTRACT_CLASS!>abstract<!> fun h()
+    <!ABSTRACT_FUNCTION_IN_NON_ABSTRACT_CLASS, ABSTRACT_FUNCTION_WITH_BODY!>abstract<!> fun j() {}
 
     //property accessors
     var i: Int                       abstract get  abstract set