FIR: introduce FictitiousFunctionSymbol making it not FIR-based

compiler/testData/fir/resolve/stdlib/functionX.kt

@@ -2,4 +2,8 @@ import kotlin.jvm.functions.Function0
 
 val x: Function0<Int> = { 42 }
 
-val y: Function1<String, String> = { it }
+val y: Function1<String, String> = { it }
+
+class MyFunction : Function2<Int, String, Unit> {
+    override fun invoke(p1: Int, p2: String) {}
+}

compiler/testData/fir/resolve/stdlib/functionX.txt

@@ -3,3 +3,8 @@ FILE: functionX.kt
         public? get(): R|kotlin/jvm/functions/Function0<kotlin/Int>|
     public? final? property y(val): R|kotlin/Function1<kotlin/String, kotlin/String>| = STUB
         public? get(): R|kotlin/Function1<kotlin/String, kotlin/String>|
+    public? final class MyFunction : R|kotlin/Function2<kotlin/Int, kotlin/String, kotlin/Unit>| {
+        public? open? override function invoke(p1: R|kotlin/Int|, p2: R|kotlin/String|): R|error: Not supported: FirImplicitTypeImpl| {
+        }
+
+    }